Do we need a way to go test in all repositories?

[marten-seemann]

During the OpenStream refactor, I found that it would be useful to have a way to run go test ./... in all repositories, in order to discover if the interface changes lead to unexpected build / test failures (yes, they did, a lot).

Would it make sense to add a command for this in workspace.sh? I could imagine two ways to do this:

1. Either add a workspace.sh test flag, that runs go test ./... in all repos.
2. Or add a workspace.sh exec <cmd>.

Any thoughts?