net/rpc: fix aliasing in TestAcceptExitAfterListenerClose

TestRPC writes to newServer and newServerAddr guarded with a
sync.Once.
TestAcceptExitAfterListenerClose was overwriting those variables,
which caused the second invocation of TestRPC within a single process
to fail.

A second invocation can occur as a result of running the test with
multiple values for the -cpu flag.

fixes #19001.

Change-Id: I291bacf44aefb49c2264ca0290a28248c026f80e
Reviewed-on: https://go-review.googlesource.com/36624Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/net/rpc/server_test.go

@@ -619,13 +619,13 @@ func TestErrorAfterClientClose(t *testing.T) {
 
 // Tests the fix to issue 11221. Without the fix, this loops forever or crashes.
 func TestAcceptExitAfterListenerClose(t *testing.T) {
-	newServer = NewServer()
+	newServer := NewServer()
 	newServer.Register(new(Arith))
 	newServer.RegisterName("net.rpc.Arith", new(Arith))
 	newServer.RegisterName("newServer.Arith", new(Arith))
 
 	var l net.Listener
-	l, newServerAddr = listenTCP()
+	l, _ = listenTCP()
 	l.Close()
 	newServer.Accept(l)
 }