kselftest: create fixture objects

Grouping tests by fixture will allow us to parametrize
test runs. Create full objects for fixtures.

Add a "global" fixture for tests without a fixture.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>