net/http: whitelist another non-http goroutine in leak checker

Fixes #20810

Change-Id: I09365b2db50c41aa3383dd730859b6f2cdb78e63
Reviewed-on: https://go-review.googlesource.com/46836
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/main_test.go

@@ -37,6 +37,7 @@ func interestingGoroutines() (gs []string) {
 		}
 		stack := strings.TrimSpace(sl[1])
 		if stack == "" ||
+			strings.Contains(stack, "testing.(*M).before.func1") ||
 			strings.Contains(stack, "os/signal.signal_recv") ||
 			strings.Contains(stack, "created by net.startServer") ||
 			strings.Contains(stack, "created by testing.RunTests") ||