http2: fix race in TestCanonicalHeaderCacheGrowth

As Damien found in the 2nd patch set of CL 455736 and CL 455735,
disableGoroutineTracking is racy (but only called by benchmarks
that don't care); set the serverConn.serveG in the test instead.

Fixes golang/go#57218.

Change-Id: I5e39be3c7fbac93260a26951491863b06b39d672
Reviewed-on: https://go-review.googlesource.com/c/net/+/456523
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

http2/server_test.go

@@ -4547,10 +4547,11 @@ func TestServerInitialFlowControlWindow(t *testing.T) {
 // TestCanonicalHeaderCacheGrowth verifies that the canonical header cache
 // size is capped to a reasonable level.
 func TestCanonicalHeaderCacheGrowth(t *testing.T) {
-	defer disableGoroutineTracking()()
 	for _, size := range []int{1, (1 << 20) - 10} {
 		base := strings.Repeat("X", size)
-		sc := &serverConn{}
+		sc := &serverConn{
+			serveG: newGoroutineLock(),
+		}
 		const count = 1000
 		for i := 0; i < count; i++ {
 			h := fmt.Sprintf("%v-%v", base, i)