Make the currently-skipped known-failing test actually fail.

Before it was marked as skipped until we could fix it, but if you
forced it to run, it should print out a panic stack trace and then say
PASS. So this makes it actually fail.

Actual fix coming later, now that we have an actual failing list.

server.go

@@ -55,6 +55,7 @@ var responseWriterStatePool = sync.Pool{
 var (
 	testHookOnConn        func()
 	testHookGetServerConn func(*serverConn)
+	testHookOnPanic       func(sc *serverConn, panicVal interface{}) (rePanic bool)
 )
 
 // TODO: finish GOAWAY support. Consider each incoming frame type and
@@ -450,8 +451,19 @@ func (sc *serverConn) stopShutdownTimer() {
 	}
 }
 
+func (sc *serverConn) notePanic() {
+	if testHookOnPanic != nil {
+		if e := recover(); e != nil {
+			if testHookOnPanic(sc, e) {
+				panic(e)
+			}
+		}
+	}
+}
+
 func (sc *serverConn) serve() {
 	sc.serveG.check()
+	defer sc.notePanic()
 	defer sc.conn.Close()
 	defer sc.closeAllStreamsOnConnClose()
 	defer sc.stopShutdownTimer()

server_test.go

@@ -42,6 +42,7 @@ type serverTester struct {
 }
 
 func newServerTester(t *testing.T, handler http.HandlerFunc) *serverTester {
+	testHookOnPanic = nil
 	logBuf := new(bytes.Buffer)
 	ts := httptest.NewUnstartedServer(handler)
 	ConfigureServer(ts.Config, &Server{})
@@ -1793,6 +1794,7 @@ func TestServer_Response_ManyHeaders_With_Continuation(t *testing.T) {
 
 func TestServer_NoCrash_HandlerClose_Then_ClientClose(t *testing.T) {
 	condSkipFailingTest(t)
+
 	testServerResponse(t, func(w http.ResponseWriter, r *http.Request) error {
 		// nothing
 		return nil
@@ -1813,10 +1815,28 @@ func TestServer_NoCrash_HandlerClose_Then_ClientClose(t *testing.T) {
 		// it doesn't crash with an internal invariant panic, like
 		// it did before.
 		st.writeData(1, true, []byte("foo"))
+
+		var (
+			panMu    sync.Mutex
+			panicVal interface{}
+		)
+		testHookOnPanic = func(sc *serverConn, pv interface{}) bool {
+			panMu.Lock()
+			panicVal = pv
+			panMu.Unlock()
+			return true
+		}
+
 		st.cc.Close()
 		select {
 		case <-st.sc.doneServing:
 			// Loop has exited.
+			panMu.Lock()
+			got := panicVal
+			panMu.Unlock()
+			if got != nil {
+				t.Errorf("Got panic: %v", got)
+			}
 		case <-time.After(5 * time.Second):
 			t.Error("timeout")
 		}