kselftest: arm64: extend test_init functionalities

Extend signal testing framework to allow the definition of a custom per
test initialization function to be run at the end of the common test_init
after test setup phase has completed and before test-run routine.

This custom per-test initialization function also enables the test writer
to decide on its own when forcibly skip the test itself using standard KSFT
mechanism.

Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>