net/http: fix race on postPendingDial test hook

The race occurs rarely, but by putting some delays and more reads/writes
of prePendingDial/postPendingDial in the handlePendingDial function I
could reproduce it.

Fixes #11136

Change-Id: I8da9e66c88fbda049eaaaaffa2717264ef327768
Reviewed-on: https://go-review.googlesource.com/11250Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/transport.go

@@ -523,6 +523,11 @@ func (t *Transport) getConn(req *Request, cm connectMethod) (*persistConn, error
 	}
 	dialc := make(chan dialRes)
 
+	// Copy these hooks so we don't race on the postPendingDial in
+	// the goroutine we launch. Issue 11136.
+	prePendingDial := prePendingDial
+	postPendingDial := postPendingDial
+
 	handlePendingDial := func() {
 		if prePendingDial != nil {
 			prePendingDial()