Create an extra variation in `test_per_src` mutator collecting all outputs.

Have `cc.testPerSrcMutator` create an additional variation named "",
having no sources (and generating no output file), but depending on
all other `test_per_src` variations and collecting their output files
in a new field named `cc.Module.testPerSrcOutputFiles`. This is useful
in the case where a module depends on all the `test_per_src`
variations of a test module.

Test: m
Bug: 129534335
Change-Id: I905decc0b9417f47cee9113466677d3bb61ad7b6