string_kunit: Add test cases for str*cmp functions

Currently, str*cmp functions (strcmp, strncmp, strcasecmp and
strncasecmp) are not covered with tests. Extend the `string_kunit.c`
test by adding the test cases for them.

This patch adds 8 more test cases:

1) strcmp test
2) strcmp test on long strings (2048 chars)
3) strncmp test
4) strncmp test on long strings (2048 chars)
5) strcasecmp test
6) strcasecmp test on long strings
7) strncasecmp test
8) strncasecmp test on long strings

These test cases aim at covering as many edge cases as possible,
including the tests on empty strings, situations when the different
symbol is placed at the end of one of the strings, etc.

Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Link: https://lore.kernel.org/r/20240417233033.717596-1-ivan.orlov0322@gmail.com


Signed-off-by: Kees Cook <keescook@chromium.org>