Override IO transform for JobTest

[mkuthan]

1. Scio CustomOutput doesn't work because it returns ClosedTap and we need dead letters.
2. TransformOverride is a hack but how to easily test output?
3. In general all methods in the infrastructure module should be implemented as single transformation, easy to stub in the job tests.

[mkuthan]

Done by extending Scio API locally but it should be accepted by Scio maintainers, see:
spotify/scio#4995