Add HandlerConfigurationApiFake test helper class

The new class is a copy of the SignalNumberSetFake and SigactionApiFake
classes, which will be removed soon in favor of the new class.
Add TimeApiFake test helper class
Add HandlerConfigurationApiDummy test helper class
Add PselectApiStub test helper class
Use new test helpers for Awaiter tests

The new test doubles do not depend on irrelevant API classes.
Remove obsolete API test helper classes