Many of our test cases only use a single NamedInstance from the
`servers` fixture. Introduce `nsX` helper fixtures to simplify these
tests and reduce boilterplate code further.
Specifically, the test no longer has to either define its own variable
to extract a single server from the list, or use the longer
servers["nsX"] syntax. While this may seem minor, the amount of times it
is repeated across the tests justifies the change. It also promotes
using more explicit server identification, i.e. `nsX`, rather than
generic `server`. This also improves the clarity of the tests and may be
helpful in traceback during debugging as well.
