Add unit tests for installation helpers

[mwoehlke-kitware]

#6072 adds some internal helper functions that probably should have some unit tests (strip_prefix in particular). For that matter, it would be nice to unit test install itself if the framework lets us do that. (Inspecting the returned actions for sanity should be sufficient, and can be done against a test project.)

[jwnimmer-tri]

Closing as duplicate of #7331.