kselftest: run tests by fixture

Now that all tests have a fixture object move from a global
list of tests to a list of tests per fixture.

Order of tests may change as we will now group and run test
fixture by fixture, rather than in declaration order.

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