selftests/harness: Constify fixture variants

FIXTURE_VARIANT_ADD() types are passed as const pointers to
FIXTURE_TEARDOWN(). Make that explicit by constifying the variants
declarations.

Cc: Shuah Khan <[e-mail]>
Cc: Will Drewry <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mickaël Salaün <[e-mail]>

tools/testing/selftests/kselftest_harness.h

···
  * variant.
  */
 #define FIXTURE_VARIANT_ADD(fixture_name, variant_name) \
-	extern FIXTURE_VARIANT(fixture_name) \
+	extern const FIXTURE_VARIANT(fixture_name) \
 		_##fixture_name##_##variant_name##_variant; \
 	static struct __fixture_variant_metadata \
 		_##fixture_name##_##variant_name##_object = \
···
 		__register_fixture_variant(&_##fixture_name##_fixture_object, \
 			&_##fixture_name##_##variant_name##_object);	\
 	} \
-	FIXTURE_VARIANT(fixture_name) \
+	const FIXTURE_VARIANT(fixture_name) \
 		_##fixture_name##_##variant_name##_variant =
 
 /**