selftests: kselftest_harness: support using xfail

Currently some tests report skip for things they expect to fail
e.g. when given combination of parameters is known to be unsupported.
This is confusing because in an ideal test environment and fully
featured kernel no tests should be skipped.

Selftest summary line already includes xfail and xpass counters,
e.g.:

  Totals: pass:725 fail:0 xfail:0 xpass:0 skip:0 error:0

but there's no way to use it from within the harness.

Add a new per-fixture+variant combination list of test cases
we expect to fail.

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