ext4: add Kunit coverage for directory hash computation

Introduce Kunit tests for fs/ext4/hash.c to verify ext4fs_dirhash()
across the legacy, half-MD4, and TEA hash variants.

The tests cover empty, seeded hashing, and non-ASCII name handling.
They also verify error paths, including invalid hash versions and
SipHash without a configured key, and check that the signed and
unsigned hash variants differ on non-ASCII input as expected.

When CONFIG_UNICODE is enabled, the tests further verify casefolded-name
hashing and the fallback behavior for invalid input.

Co-developed-by: Chen Hao Yu <edward062254@gmail.com>
Signed-off-by: Chen Hao Yu <edward062254@gmail.com>
Signed-off-by: Guan-Chun Wu <409411716@gms.tku.edu.tw>
Link: https://patch.msgid.link/20260531080019.3794809-2-409411716@gms.tku.edu.tw
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

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