kselftest/arm64: signal: Skip tests if required features are missing

kselftest/arm64: signal: Skip tests if required features are missing

During initialization of a signal testcase, features declared as required
are properly checked against the running system but no action is then taken
to effectively skip such a testcase.

Fix core signals test logic to abort initialization and report such a
testcase as skipped to the KSelfTest framework.

Fixes: f96bf4340316 ("kselftest: arm64: mangle_pstate_invalid_compat_toggle and common utils")
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20210920121228.35368-1-cristian.marussi@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>