Make test charm repository available from testing package
charm, state, and store were all testing against the same repo, and very soon so will hook (so it can have a real unit to work with); seemed sensible to make it hard for tests to accidentally write to charms they shouldn't, and to avoid explicit use of "../charm/testrepo" in other packages.
Only notable change is file mode testing; it was actually only tested in one place, and implicitly at that, so it's now explicitly tested in its own test.