[4/8] Use assert(Not)Regex instead of assertTrue or assertFalse in tests

[Flamefire]

This makes the tests shorter and hence easier to read, avoids re.compile calls and manual failure message composing. In some cases assert_multi_regex could be used to avoid the explicit loops.

[Flamefire]

@boegel I just had to fix a test:
self.assert_multi_regex automatically considers the passed patterns as multi-line enabled. But the test intended to check for whitespace at the beginning and end of the whole string only.

This begs the question if the default of multi_line=True is correct: It works (now) for all checks, most are simple patterns not affected by that setting anyway. But it seems the better default would be False matching the semantics of assert(Not)Regex
But I simply copied the existing _assert_regexs function from test/framework/options.py which had this default.

[akesandgren]

LGTM

[akesandgren]

Going in, thanks @Flamefire!