net/http: skip TestTransportPersistConnLeak/h2

We started running this test under HTTP/2 in a recent refactoring.
It seems to be flaky for HTTP/2; skip it for now.

Change-Id: I8b270afe7f0d3db307b5a951e16f576116333003
Reviewed-on: https://go-review.googlesource.com/c/go/+/443075
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Damien Neil <dneil@google.com>

src/net/http/transport_test.go

@@ -1680,6 +1680,9 @@ func TestTransportPersistConnLeak(t *testing.T) {
 	run(t, testTransportPersistConnLeak, testNotParallel)
 }
 func testTransportPersistConnLeak(t *testing.T, mode testMode) {
+	if mode == http2Mode {
+		t.Skip("flaky in HTTP/2")
+	}
 	// Not parallel: counts goroutines
 
 	const numReq = 25