Integration tests: Use BeforeTest(t) instead of defer AfterTest().

Thanks to this change, a single method BeforeTest(t) can handle
before-test logic as well as registration of cleanup code
(t.Cleanup(func)).

server/etcdserver/api/rafthttp/stream_test.go

@@ -187,7 +187,7 @@ func TestStreamReaderDialResult(t *testing.T) {
 
 // TestStreamReaderStopOnDial tests a stream reader closes the connection on stop.
 func TestStreamReaderStopOnDial(t *testing.T) {
-	defer testutil.AfterTest(t)
+	testutil.BeforeTest(t)
 	h := http.Header{}
 	h.Add("X-Server-Version", version.Version)
 	tr := &respWaitRoundTripper{rrt: &respRoundTripper{code: http.StatusOK, header: h}}