selftests: harness: Ignore unused variant argument warning

For tests without fixtures the variant argument is unused.
This is intentional, prevent to compiler from complaining.

Example warning:

    harness-selftest.c: In function 'wrapper_standalone_pass':
    ../kselftest_harness.h:181:52: error: unused parameter 'variant' [-Werror=unused-parameter]
      181 |                 struct __fixture_variant_metadata *variant) \
          |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~
    ../kselftest_harness.h:156:25: note: in expansion of macro '__TEST_IMPL'
      156 | #define TEST(test_name) __TEST_IMPL(test_name, -1)
          |                         ^~~~~~~~~~~
    harness-selftest.c:15:1: note: in expansion of macro 'TEST'
       15 | TEST(standalone_pass) {
          | ^~~~

Signed-off-by: Thomas Weißschuh <[email protected]>
Reviewed-by: Muhammad Usama Anjum <[email protected]>
Acked-by: Shuah Khan <[email protected]>
Link: https://lore.kernel.org/r/20250505-nolibc-kselftest-harness-v4-3-ee4dd5257135@linutronix.de
Signed-off-by: Thomas Weißschuh <[email protected]>

Author: t-8ch

tools/testing/selftests/kselftest_harness.h

@@ -174,7 +174,7 @@
 	static void test_name(struct __test_metadata *_metadata); \
 	static inline void wrapper_##test_name( \
 		struct __test_metadata *_metadata, \
-		struct __fixture_variant_metadata *variant) \
+		struct __fixture_variant_metadata __attribute__((unused)) *variant) \
 	{ \
 		_metadata->setup_completed = true; \
 		if (setjmp(_metadata->env) == 0) \