Add link dependency on test package archive

The test package archive was being picked up through "-L" + testRoot,
but did not have a dependency on it.  If the *_test.go file was
changed but not the implementation then the tests would not be
rerun.

Test: Add t.Fatal() to a *_test.go file and rebuild.
Change-Id: I32b8c7a0fa1d5b76b1242646887fe937e396e318