Update testNs to take a collection of namespaces

[bpringe]

We should probably allow testNs to take an array of namespaces (and change its name to something like testNamespaces) since the ns-query's exactly prop can take a collection of namespaces. We call it in a mapping of namespaces in one place and return a collection of promises, but we could just call it once and pass the collection of namespaces.

This is insignificant but was noticed while working on another PR, so I'm creating this issue to track it.