internal_dns tests don't clean up after themselves

[davepacheco]

It looks like the internal DNS tests spawn a separate tokio task to run the DNS server and never clean this up. They also don't clean up the files they create in $TMPDIR (which is hard to fix because the server is still running). This latter part makes it hard for us to write a check that no tests leave detritus in $TMPDIR, which makes it easy to create more tests that leave stuff there.

[davepacheco]

@rcgoodfellow, is this something you could look into? It's possible the patterns we use for Nexus integration tests will be useful here. They have a "test context" object that has a cleanup function that tears things down. We also wrote a proc macro to make it easier to use so you don't need to remember to call the cleanup function. All this might be overkill here though.