net/http: change TestServerAllowsBlockingRemoteAddr to non-parallel

It appears that this test is particularly
sensitive to resource starvation.
Returning it to non-parallel should reduce flakiness,
by giving it the full system resources to run.

Fixes #19161

Change-Id: I6e8906516629badaa0cffeb5712af649dc197f39
Reviewed-on: https://go-review.googlesource.com/38005
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/serve_test.go

@@ -929,7 +929,6 @@ func (c *blockingRemoteAddrConn) RemoteAddr() net.Addr {
 
 // Issue 12943
 func TestServerAllowsBlockingRemoteAddr(t *testing.T) {
-	setParallel(t)
 	defer afterTest(t)
 	ts := httptest.NewUnstartedServer(HandlerFunc(func(w ResponseWriter, r *Request) {
 		fmt.Fprintf(w, "RA:%s", r.RemoteAddr)