Make the tests pass on a busy host (closes #240)

Tests that waited a few seconds for something to happen now wait 15 seconds.
The wait ends as soon as the condition holds, so a passing run takes no longer.

TestStreamNotDroppedWhileLinesArrive runs on a fake clock, so a stall on the
host cannot use up its idle timeout.

The checkpoint tests give the writing connection a 30-second busy timeout, so
their limit can be 15 seconds and a checkpoint that waits for a read still
fails them.

Model: opus-5-5

internal/server/handlers_internal_test.go

@@ -1397,8 +1397,11 @@ func TestIPLookupKeepsRouteWhenPTRLookupDoesNotAnswer(t *testing.T) {
 	s := newServerWithTestRoutes(t)
 	s.resolver = &fakeResolver{ptrNeverAnswers: true}
 
-	// The request's time limit passes after the PTR lookup's.
-	ctx, cancel := context.WithTimeout(t.Context(), 2*ptrLookupTimeout)
+	// The request's time limit passes well after the PTR lookup's, so that a
+	// stall on a busy host does not use up the difference.
+	const requestTimeLimit = ptrLookupTimeout + 15*time.Second
+
+	ctx, cancel := context.WithTimeout(t.Context(), requestTimeLimit)
 	defer cancel()
 
 	rec := httptest.NewRecorder()
@@ -2226,9 +2229,13 @@ func settledGoroutineCount() int {
 	return prev
 }
 
-// waitForGoroutines waits until the goroutine count drops to target or below.
+// waitForGoroutines waits up to 15 seconds for the goroutine count to drop to
+// target or below.
 func waitForGoroutines(target int) bool {
-	for range 100 {
+	const maxWait = 15 * time.Second
+
+	deadline := time.Now().Add(maxWait)
+	for time.Now().Before(deadline) {
 		if runtime.NumGoroutine() <= target {
 			return true
 		}