selftests/bpf: Check whether to run selftest

The sockopt test invokes test__start_subtest and then unconditionally
asserts the success. That means that even if deny-listed, any test will
still run and potentially fail.
Evaluate the return value of test__start_subtest() to achieve the
desired behavior, as other tests do.

Signed-off-by: Daniel Müller <deso@posteo.net>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230525232248.640465-1-deso@posteo.net