Require testrepository>=0.0.18

To help with debugging, quoting from upstream:

 `run` now accepts `--isolated` as a parameter, which will cause
 each selected test to be run independently. This can be useful
 to both workaround isolation bugs and detect tests that can
 not be run independently.

Change-Id: I168700457566067cc7389d290a2d7ef4e87ba8a2
Merge "Require testrepository>=0.0.18"