Go: augment test-extraction tests

[smowton]

This adds a case exercising the need for packages to be visited in topological order (cross-package type variable references), and a _test package that wouldn't exist in go list output without the -test flag. Unfortunately I couldn't reproduce the circumstance where that causes a crash, but by instrumenting the code that invokes go list I can see that with the -test flag we get everything we need out of one go list run, whereas without it we fall back to the slow path that issues individual go list queries on a per package basis.