Go: use coverprofile with multiple packages when upgrading to Go 1.10

[smola]

This is just a reminder that, once we drop support for Go <1.10, we can simplify the test-coverage rule to do a single go test call for all packages.

See golang/go#6909

[smola]

Go 1.11 is now released, so we can drop 1.9 support and this change could be done.