hfs: introduce KUnit tests for HFS string operations

hfs: introduce KUnit tests for HFS string operations

This patch implements the initial Kunit based set of
unit tests for HFS string operations. It checks
functionality of hfs_strcmp(), hfs_hash_dentry(),
and hfs_compare_dentry() methods.

./tools/testing/kunit/kunit.py run --kunitconfig ./fs/hfs/.kunitconfig

[16:04:50] Configuring KUnit Kernel ...
Regenerating .config ...
Populating config with:
$ make ARCH=um O=.kunit olddefconfig
[16:04:51] Building KUnit Kernel ...
Populating config with:
$ make ARCH=um O=.kunit olddefconfig
Building with:
$ make all compile_commands.json scripts_gdb ARCH=um O=.kunit --jobs=22
[16:04:59] Starting KUnit Kernel (1/1)...
[16:04:59] ============================================================
Running tests with:
$ .kunit/linux kunit.enable=1 mem=1G console=tty kunit_shutdown=halt
[16:04:59] ================= hfs_string (3 subtests) ==================
[16:04:59] [PASSED] hfs_strcmp_test
[16:04:59] [PASSED] hfs_hash_dentry_test
[16:04:59] [PASSED] hfs_compare_dentry_test
[16:04:59] =================== [PASSED] hfs_string ====================
[16:04:59] ============================================================
[16:04:59] Testing complete. Ran 3 tests: passed: 3
[16:04:59] Elapsed time: 9.087s total, 1.310s configuring, 7.611s building, 0.125s running

v2
Fix linker error.

v3
Chen Linxuan suggested to use EXPORT_SYMBOL_IF_KUNIT.

Signed-off-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
cc: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
cc: Yangtao Li <frank.li@vivo.com>
cc: linux-fsdevel@vger.kernel.org
cc: Chen Linxuan <me@black-desk.cn>
Reviewed-by: Chen Linxuan <me@black-desk.cn>
Signed-off-by: Viacheslav Dubeyko <slava@dubeyko.com>
Link: https://lore.kernel.org/r/20250912225022.1083313-1-slava@dubeyko.com
Signed-off-by: Viacheslav Dubeyko <slava@dubeyko.com>