resolved: Allow test-resolved-stream to run concurrently

resolved: Allow test-resolved-stream to run concurrently

Since test-resolved-stream brings up a simple DNS server on 127.0.0.1:12345,
only one instance could run at a time, so it would fail when run like
`meson test -C build test-resolved-stream --repeat=1000`.
Similarly, if by chance something is up on port 12345, the test would fail.

To make the test more reliable, run it in an isolated user + network namespace.
If this fails (some distributions disable user namespaces), just run as before.