test: fix TestBackupTier2Restore_DoubleClickRefused scheduling flake

IsRunning flips before the restore goroutine parks in StopStack; under parallel
load the ==1 assertion raced it and read 0. Wait for the first stop to land, then
assert — the red-proof property (no-fast-path mutant → count 2) is preserved.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PSK5g6qYLknKj8u3QAFEr6

controller/internal/web/async_restore_test.go

@@ -148,6 +148,9 @@ func TestBackupTier2Restore_DoubleClickRefused(t *testing.T) {
 	if loc := w2.Header().Get("Location"); !strings.Contains(loc, "m%C3%A1r+fut") && !strings.Contains(loc, "már fut") {
 		t.Fatalf("second POST should be refused with 'már fut'; got %q", loc)
 	}
+	// IsRunning flips before the goroutine reaches StopStack — wait for the FIRST stop to land, so
+	// the ==1 assertion below measures "no second launch", not goroutine scheduling (parallel-load flake).
+	waitFor(t, func() bool { return atomic.LoadInt32(&prov.stops) >= 1 }, "first restore reached StopStack")
 	if got := atomic.LoadInt32(&prov.stops); got != 1 {
 		t.Fatalf("double-click launched a second restore: StopStack count = %d, want 1", got)
 	}