lib/kunit: Optimize calls to igt_success/skip/fail()

Calling igt_success() explicitly at the end of subtest body is not needed.
Other calls to igt_success() can be usually avoided by inverting test
result checks.  Optimize the code that now calls igt_success().

Moreover, using more advanced variants of igt_skip() and igt_fail() where
applicable makes the code more compact.  Go for it.

Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab@kernel.org>