Allow `test` command to take an optional namespace argument

[pchiusano]

So test .foo.bar runs all the tests in that namespace. Rather than needing to cd .foo.bar then test with no args.

[rlmark]

I just ran into this as a result of fielding challenges to the Exercism track workflow, +1! Now that we have scoped namespaces, it's most common for folks to work high in their namespace tree with the lib directory present (though, I understand this will change in the near future with projects work); in any case, it would be nice not to have to cd down the namespace tree to run tests.