Applier unit test cleanup

- Add test for applier.Run context cancel/timeout
- Add AssertEqual & AssertNotEqual to wrap testutil.Equal matcher
- Move common test utils to common_test.go
- Use yaml artifacts, instead of Unstructured
- Use apply.Options as test input
- Use UnstructuredSet as test input
- Extract Applier construction with fake inputs to newTestApplier
- Extract nil inventory to its own test to simplify test inputs
  to TestReadAndPrepareObjects