test-only packages are not run on save

[zmb3]

If a package contains only tests (ie all files end in _test.go), go-plus does not correctly run tests on save. While this isn't super common, you occasionally run into packages like this for integration tests.

The reason this doesn't work is because our save event handler runs go build prior to go test, and doesn't run tests if go build fails. It looks like go build returns a non-zero exit code in this case, which looks just like a compiler error.

We could detect this case by looking at go build's stderr and searching for no non-test Go files in.