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 <skhan@linuxfoundation.org>
Cc: Will Drewry <wad@chromium.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20240511171445.904356-7-mic@digikod.net


Signed-off-by: Mickaël Salaün <mic@digikod.net>