quic: skip leaked goroutine check on GOOS=js

Fixes golang/go#64620

Change-Id: I3b5ff4d1e1132a47b7cc7eb00861e9f7b76f8764
Reviewed-on: https://go-review.googlesource.com/c/net/+/548455
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

internal/quic/main_test.go

@@ -22,6 +22,11 @@ func TestMain(m *testing.M) {
 	//
 	// Checking after every test makes it easier to tell which test is the culprit,
 	// but checking once at the end is faster and less likely to miss something.
+	if runtime.GOOS == "js" {
+		// The js-wasm runtime creates an additional background goroutine.
+		// Just skip the leak check there.
+		return
+	}
 	start := time.Now()
 	warned := false
 	for {