lib/uuid_kunit: add tests for the four random UUID/GUID generators

lib/uuid_kunit: add tests for the four random UUID/GUID generators

uuid_kunit currently exercises only guid_parse() and uuid_parse() (plus
their invalid-input paths).  The four random generators exported from
lib/uuid.c -- generate_random_uuid(), generate_random_guid(), uuid_gen()
and guid_gen() -- have no direct kunit coverage.

Random output cannot be compared against a fixed expected value, but RFC
4122 section 4.4 specifies two invariants that any version-4 random
UUID/GUID must satisfy:

  - version 4 in the high nibble of the version byte
    (byte 6 in the wire uuid_t layout, byte 7 in the byte-swapped
    guid_t layout);
  - variant DCE 1.1 (binary 10x) in the high bits of byte 8.

Add four test cases that invoke each generator several times and verify
these bit patterns hold.  The same checks catch a regression in either the
mask/OR sequence in the generators or the layout constants.  Run the loop
a handful of times to cover the small but non-zero chance that an unmasked
random byte happens to satisfy the version/variant pattern by accident on
a single call.

Link: https://lore.kernel.org/20260516120915.40544-1-sozdayvek@gmail.com
Signed-off-by: Stepan Ionichev <sozdayvek@gmail.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Brendan Higgins <brendan.higgins@linux.dev>
Cc: David Gow <david@davidgow.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>