kselftest: add fixture variants

Allow users to build parameterized variants of fixtures.

If fixtures want variants, they call FIXTURE_VARIANT() to declare
the structure to fill for each variant. Each fixture will be re-run
for each of the variants defined by calling FIXTURE_VARIANT_ADD()
with the differing parameters initializing the structure.

Since tests are being re-run, additional initialization (steps,
no_print) is also added.

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