Add test file for testing file locators.

Refactored the file locator implementation to allow for mocking of the file
system.  Realized I was missing a few bits of functionality from the
CL.Locator type, which are mostly (but not fully, see needs-compile)
done.

Also added a use of compiler/locators/file.arr in initialize-trove so it gets
pulled in.

The test isn't 100% working, but I'm not sure what it's choking on at the
moment, may need to check in with Joe and see what's up here tomorrow.