…lkey-io#3174)

I was working on ASAN large memory tests when I countered this issue.

The issue was that the hardcoded `999` key could land in an early
bucket. Then shrink rehash could finish early, and later inserts could
trigger a new expansion rehash, resetting rehash_idx low. The test now
picks the survivor key dynamically as the key mapped to the highest
bucket index.

```
[test_hashtable.c] Memory leak detected of 336 bytes
=================================================================
==3901==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 80 byte(s) in 1 object(s) allocated from:
    #0 0x7fb0556fd9c7 in malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:69
    #1 0x563bfdf4c47d in ztrymalloc_usable_internal /home/runner/work/valkey/valkey/src/zmalloc.c:156
    #2 0x563bfdf4c47d in valkey_malloc /home/runner/work/valkey/valkey/src/zmalloc.c:185
    #3 0x563bfdd42eaf in hashtableCreate /home/runner/work/valkey/valkey/src/hashtable.c:1217
    #4 0x563bfdaa1cbf in test_empty_buckets_rehashing unit/test_hashtable.c:232
    #5 0x563bfdae772b in runTestSuite unit/test_main.c:36
    valkey-io#6 0x563bfda86b20 in main unit/test_main.c:108
    valkey-io#7 0x7fb05522a1c9  (/lib/x86_64-linux-gnu/libc.so.6+0x2a1c9) (BuildId: 274eec488d230825a136fa9c4d85370fed7a0a5e)
    valkey-io#8 0x7fb05522a28a in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28a) (BuildId: 274eec488d230825a136fa9c4d85370fed7a0a5e)
    valkey-io#9 0x563bfda8a5c4 in _start (/home/runner/work/valkey/valkey/src/valkey-unit-tests+0x17c5c4) (BuildId: 44cfc183e6e82e499bcc9f6adc094d7f774ee9d2)

Indirect leak of 128 byte(s) in 1 object(s) allocated from:
    #0 0x7fb0556fd340 in calloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:77
    #1 0x563bfdf4c922 in ztrycalloc_usable_internal /home/runner/work/valkey/valkey/src/zmalloc.c:214
    #2 0x563bfdf4c922 in valkey_calloc /home/runner/work/valkey/valkey/src/zmalloc.c:257
    #3 0x563bfdd40967 in resize /home/runner/work/valkey/valkey/src/hashtable.c:741
    #4 0x563bfdd45eb1 in hashtableExpandIfNeeded /home/runner/work/valkey/valkey/src/hashtable.c:1446
    #5 0x563bfdd45eb1 in hashtableExpandIfNeeded /home/runner/work/valkey/valkey/src/hashtable.c:1433
    valkey-io#6 0x563bfdd45eb1 in insert /home/runner/work/valkey/valkey/src/hashtable.c:1041
    valkey-io#7 0x563bfdd45eb1 in hashtableAddOrFind /home/runner/work/valkey/valkey/src/hashtable.c:1554
    valkey-io#8 0x563bfdd45eb1 in hashtableAdd /home/runner/work/valkey/valkey/src/hashtable.c:1539
    valkey-io#9 0x563bfdaa1e3b in test_empty_buckets_rehashing unit/test_hashtable.c:254
    valkey-io#10 0x563bfdae772b in runTestSuite unit/test_main.c:36
    valkey-io#11 0x563bfda86b20 in main unit/test_main.c:108
    valkey-io#12 0x7fb05522a1c9  (/lib/x86_64-linux-gnu/libc.so.6+0x2a1c9) (BuildId: 274eec488d230825a136fa9c4d85370fed7a0a5e)
    valkey-io#13 0x7fb05522a28a in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28a) (BuildId: 274eec488d230825a136fa9c4d85370fed7a0a5e)
    valkey-io#14 0x563bfda8a5c4 in _start (/home/runner/work/valkey/valkey/src/valkey-unit-tests+0x17c5c4) (BuildId: 44cfc183e6e82e499bcc9f6adc094d7f774ee9d2)

Indirect leak of 64 byte(s) in 1 object(s) allocated from:
    #0 0x7fb0556fd340 in calloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:77
    #1 0x563bfdf4c922 in ztrycalloc_usable_internal /home/runner/work/valkey/valkey/src/zmalloc.c:214
    #2 0x563bfdf4c922 in valkey_calloc /home/runner/work/valkey/valkey/src/zmalloc.c:257
    #3 0x563bfdd3f553 in bucketConvertToChained /home/runner/work/valkey/valkey/src/hashtable.c:908
    #4 0x563bfdd3f553 in findBucketForInsert /home/runner/work/valkey/valkey/src/hashtable.c:1021
    #5 0x563bfdd45d9e in insert /home/runner/work/valkey/valkey/src/hashtable.c:1045
    valkey-io#6 0x563bfdd45d9e in hashtableAddOrFind /home/runner/work/valkey/valkey/src/hashtable.c:1554
    valkey-io#7 0x563bfdd45d9e in hashtableAdd /home/runner/work/valkey/valkey/src/hashtable.c:1539
    valkey-io#8 0x563bfdaa1e3b in test_empty_buckets_rehashing unit/test_hashtable.c:254
    valkey-io#9 0x563bfdae772b in runTestSuite unit/test_main.c:36
    valkey-io#10 0x563bfda86b20 in main unit/test_main.c:108
    valkey-io#11 0x7fb05522a1c9  (/lib/x86_64-linux-gnu/libc.so.6+0x2a1c9) (BuildId: 274eec488d230825a136fa9c4d85370fed7a0a5e)
    valkey-io#12 0x7fb05522a28a in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28a) (BuildId: 274eec488d230825a136fa9c4d85370fed7a0a5e)
    valkey-io#13 0x563bfda8a5c4 in _start (/home/runner/work/valkey/valkey/src/valkey-unit-tests+0x17c5c4) (BuildId: 44cfc183e6e82e499bcc9f6adc094d7f774ee9d2)

Indirect leak of 64 byte(s) in 1 object(s) allocated from:
    #0 0x7fb0556fd340 in calloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:77
    #1 0x563bfdf4c922 in ztrycalloc_usable_internal /home/runner/work/valkey/valkey/src/zmalloc.c:214
    #2 0x563bfdf4c922 in valkey_calloc /home/runner/work/valkey/valkey/src/zmalloc.c:257
    #3 0x563bfdd40967 in resize /home/runner/work/valkey/valkey/src/hashtable.c:741
    #4 0x563bfdaa1df8 in test_empty_buckets_rehashing unit/test_hashtable.c:248
    #5 0x563bfdae772b in runTestSuite unit/test_main.c:36
    valkey-io#6 0x563bfda86b20 in main unit/test_main.c:108
    valkey-io#7 0x7fb05522a1c9  (/lib/x86_64-linux-gnu/libc.so.6+0x2a1c9) (BuildId: 274eec488d230825a136fa9c4d85370fed7a0a5e)
    valkey-io#8 0x7fb05522a28a in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28a) (BuildId: 274eec488d230825a136fa9c4d85370fed7a0a5e)
    valkey-io#9 0x563bfda8a5c4 in _start (/home/runner/work/valkey/valkey/src/valkey-unit-tests+0x17c5c4) (BuildId: 44cfc183e6e82e499bcc9f6adc094d7f774ee9d2)

SUMMARY: AddressSanitizer: 336 byte(s) leaked in 4 allocation(s).
```

Signed-off-by: Sarthak Aggarwal <[email protected]>