Refactor /Fo tests
Refactor /Fo tests

Add helper test _testFo() to simplify testing multiple input values for
the /Fo parameter without duplicating irrelevant noise.

This extracts a test testOutputFileFromSourcefile() because in this
case, no /Fo argument is given.

This commit does not change any functionality or test coverage.

webmaster128/clcache,webmaster128/clcache,webmaster128/clcache