selftests/harness: allow tests to be skipped during setup

Before executing each test from a fixture, FIXTURE_SETUP is run once.
When SKIP is used in FIXTURE_SETUP, the setup function returns early
but the test still proceeds to run, unless another SKIP macro is used
within the test definition, leading to some code repetition. Therefore,
allow tests to be skipped directly from the setup function.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Magali Lemes <magali.lemes@canonical.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>