Currently we copy all xpcshell tests + support files to a parallel hierarchy in the objdir, then run them from there. Realistically, on tinderbox we run everything from the test package. I think with a little bit of fiddling we could make it so we only copy some supporting files to the objdir for certain tests, and run tests from the srcdir when running tests on developer machines, and continue to mash srcdir+objdir into a single hierarchy for the test package.

gps: you were talking about this on IRC the other day. I knew I had thought about it previously.