MOBI: do not take the region of a different language in mobi2iana - #3453

mobi2iana() sets the region suffix whenever any language's entry in IANA_MOBI has the requested sub language number, not only the entries of the requested language:

if cc == langcode:
    prefix = code
if cl == sublangcode:
    suffix = subcode.lower() if subcode else None
    break

If the requested language has no entry for that sub language, suffix keeps the value from an earlier language. For example mobi2iana(7, 4) returns 'de-sa' (German with the Saudi Arabia region from Arabic's (1, 4)) instead of 'de', and (25, 8) gives 'ru-pt', (26, 12) 'hr-ca'. This moves the sub language check inside the language match. Entries that exist are unchanged: (9, 4) is still 'en-us', (7, 8) 'de-ch', (4, 12) 'zh-hk'.

I added test_mobi2iana to TestLocaleNames in src/calibre/utils/localization_test.py, since that module's find_tests is already registered.

I did not touch the caller in mobi/reader/headers.py, which passes the plain sub language while the table stores it shifted by two bits. That looks like a separate question.

I couldn't run calibre locally, so I ran the CI on a fork:

• Test only, without the fix: CI fails on ubuntu and Arch with AssertionError: 'de-sa' != 'de' / Failed for: 7, 4 in test_mobi2iana (OK: 969 FAIL: 1). The macOS and Windows jobs of that run were cancelled after the failure.
• Test with the fix: CI passes on ubuntu, macOS, Windows and Arch.

The commit in this PR is the squashed form of the fix branch (identical tree).