test: bound watcher live DNS and serialise packages after rebase

Rebasing this branch onto next surfaced two failures that the branch
did not have in isolation. Both come from the change this PR makes:
the watcher package now queries live DNS, and it is the second package
to do so.

Burst fan-out in the watcher package. All 13 watcher tests are
parallel and each runs a full iterative resolution, so they hit the
same root servers in the same instant and get rate-limited. This is
exactly the pathology internal/resolver/livedns_test.go was written to
prevent (9cb2c2b); that gate is package-scoped and cannot reach into
this package's test binary, so liveWatcherGate is its counterpart
here, acquired in newTestWatcher and released when the test ends.

Cross-package oversubscription. Go runs package binaries in parallel,
so with both live-DNS packages in flight their gates sum rather than
hold. The excess is rate-limited and the resolver's 8s per-attempt
deadlines expire, failing ResolveIPAddresses tests this branch never
touched. script/test now passes -p 1 so each gate is authoritative
while its package runs. Serialising is also net faster here, because
the retries it removes cost more than the lost parallelism: resolver
16-22s (was 30-36s under contention), watcher 12-13s (was 27-37s),
whole suite 39-46s against the 60s cap and the 90s -timeout backstop.

TestQueryNameserverIP_UnreachableServer is dropped rather than fixed.
It asserted that a query to an RFC 5737 documentation address comes
back non-OK with no records, which does not hold in the build
environment: that network transparently intercepts all UDP/53 traffic
regardless of destination and answers it locally, so the query returns
StatusOK with 9 real records for example.com. Verified directly with
dig @192.0.2.1 inside the build network. The mock DNSClient that used
to force this classification is what this PR removes, and a live
substitute would only be testing the sandbox's network behaviour, so
the coverage gap is recorded in a comment where the test was.

Verified: three consecutive `docker build .` runs green, after three
consecutive failures without these changes.

internal/watcher/watcher_test.go

@@ -191,12 +191,40 @@ type testDeps struct {
 	config      *config.Config
 }
 
+// liveWatcherConcurrency caps how many of this package's tests may
+// be driving live DNS at once. Every test here runs a full iterative
+// resolution, and the package is entirely parallel, so without a
+// bound all of them burst against the same root servers in the same
+// instant and get rate-limited — the fan-out pathology that
+// internal/resolver/livedns_test.go exists to prevent. That gate is
+// package-scoped and so cannot reach across into this package's test
+// binary; this is its counterpart here.
+const liveWatcherConcurrency = 3
+
+// liveWatcherGate bounds concurrent live-DNS watcher tests. It has to
+// be package scoped: the point is that every parallel test shares it.
+//
+//nolint:gochecknoglobals // package-wide live query rate limit
+var liveWatcherGate = make(chan struct{}, liveWatcherConcurrency)
+
+// acquireLiveSlot blocks until this test may run live DNS, releasing
+// the slot when the test ends.
+func acquireLiveSlot(t *testing.T) {
+	t.Helper()
+
+	liveWatcherGate <- struct{}{}
+
+	t.Cleanup(func() { <-liveWatcherGate })
+}
+
 func newTestWatcher(
 	t *testing.T,
 	cfg *config.Config,
 ) (*watcher.Watcher, *testDeps) {
 	t.Helper()
 
+	acquireLiveSlot(t)
+
 	deps := &testDeps{
 		portChecker: &mockPortChecker{},
 		tlsChecker:  &mockTLSChecker{},