18632: tests/thread_float: do not overload slow MCUs with IRQs r=benpicco a=maribu

### Contribution description

If the regular context switches are triggered too fast, slow MCUs will be able to spent little time on actually progressing in the test. This will scale the IRQ rate with the CPU clock as a crude way too keep load within limits.

### Testing procedure

The unit test should now pass on the Microduino CoreRF

```
$ make BOARD=microduino-corerf AVRDUDE_PROGRAMMER=dragon_jtag -C tests/thread_float flash test
make: Entering directory '/home/maribu/Repos/software/RIOT/tests/thread_float'
Building application "tests_thread_float" for "microduino-corerf" with MCU "atmega128rfa1".
[...]
   text	  data	   bss	   dec	   hex	filename
  12834	   520	  3003	 16357	  3fe5	/home/maribu/Repos/software/RIOT/tests/thread_float/bin/microduino-corerf/tests_thread_float.elf
avrdude -c dragon_jtag -p m128rfa1  -U flash:w:/home/maribu/Repos/software/RIOT/tests/thread_float/bin/microduino-corerf/tests_thread_float.hex
[...]
Welcome to pyterm!
Type '/exit' to exit.
READY
s
START
main(): This is RIOT! (Version: 2022.10-devel-858-g18566-tests/thread_float)
THREADS CREATED

Context switch every 3125 µs
{ "threads": [{ "name": "idle", "stack_size": 192, "stack_used": 88 }]}
{ "threads": [{ "name": "main", "stack_size": 640, "stack_used": 220 }]}
THREAD t1 start
THREAD t2 start
THREAD t3 start
t1: 141.443770
t3: 141.466810
t1: 141.443770
t3: 141.466810
t1: 141.443770
t3: 141.466810
t1: 141.443770
t3: 141.466810
t1: 141.443770
t3: 141.466810
t1: 141.443770
t3: 141.466810
t1: 141.443770

make: Leaving directory '/home/maribu/Repos/software/RIOT/tests/thread_float'
```

(~~Note: The idle thread exiting is something that should never occur. I guess the culprit may be `cpu_switch_context_exit()` messing things up when the main thread exits. But that is not directly related to what this PR aims to fix. Adding a `thread_sleep()` at the end of `main()` does indeed prevent the idle thread from exiting.~~
Update: That's expected. The idle thread stats are printed on exit of the main thread, the idle thread does not actually exit.)

### Issues/PRs references

Fixes #16908 maybe?

18950: tests/unittests: add unit tests for core_mbox r=benpicco a=maribu

### Contribution description

As the title says

### Testing procedure

The test cases are run on `native` by Murdock anyway.

### Issues/PRs references

Split out of #18949

19030: tests/periph_timer_short_relative_set: improve test r=benpicco a=maribu

### Contribution description

Reduce the number lines to output by only testing for intervals 0..15 to speed up the test.

In addition, run each test case 128 repetitions (it is still faster than before) to give some confidence the short relative set actually succeeded.

### Testing procedure

The test application should consistently fail or succeed, rather than occasionally passing.

### Issues/PRs references

None

19085: makefiles/tests/tests.inc.mk: fix test/available target r=benpicco a=maribu

### Contribution description

`dist/tools/compile_and_test_for_board/compile_and_test_for_board.py` relies on `make test/available` to check if a test if available. However, this so far did not take `TEST_ON_CI_BLACKLIST` and `TEST_ON_CI_WHITELIST` into account, resulting in tests being executed for boards which they are not available. This should fix the issue.

### Testing procedure


#### Expected to fail

```
$ make BOARD=nrf52840dk -C tests/gcoap_fileserver test/available
$ make BOARD=microbit -C tests/log_color test/available
```

(On `master`, they succeed, but fail in this PR.)

#### Expected to succeed

```
$ make BOARD=native -C tests/gcoap_fileserver test/available
$ make BOARD=nrf52840dk -C tests/pkg_edhoc_c test/available
$ make BOARD=nrf52840dk -C tests/log_color test/available
```

(Succeed in both `master` and this PR.)

### Issues/PRs references

None

Co-authored-by: Marian Buschsieweke <[email protected]>