[wptrunner] Optimize server test order (#46331)
TestEnvironment.ensure_started() synchronously polls that all servers
are started. The non-webtransport-h3 servers are often tested first and
immediately fail with "Connection refused", which puts them in the
pending list. This spins the polling loop at least once and delays
startup by at least 0.5s.
Run the webtransport-h3 server test first, which blocks 1. This
provides additional time for the non-webtransport-h3 servers to become
ready, which means the TestEnvironment often only needs to poll once.
This speeds up ensure_started() from ~2.2s -> ~1.7s and improves
wptrunner's interactive UX (https://crbug.com/339299999). No functional
change.