deflake TestRoundTripSocks5AndNewConnection unit test

The test was asserting the error from the proxy server, to avoid false
positives it was using a channel to communicate that the test has been
 ended and the error was legit.

However, the channel was executed using a defer statement, and the connection
on the server could be closed accidentally by the connection request,
that was done later and used another defer statement to close it.
Since the connection defer was done later, it was executed before the
channel close.

staging/src/k8s.io/apimachinery/pkg/util/httpstream/spdy/roundtripper_test.go

@@ -438,7 +438,6 @@ func (i *Interceptor) Rewrite(ctx context.Context, req *socks5.Request) (context
 
 // be sure to unset environment variable https_proxy (if exported) before testing, otherwise the testing will fail unexpectedly.
 func TestRoundTripSocks5AndNewConnection(t *testing.T) {
-	t.Skip("Flake https://issues.k8s.io/107708")
 	localhostPool := localhostCertPool(t)
 
 	for _, redirect := range []bool{false, true} {
@@ -568,10 +567,7 @@ func TestRoundTripSocks5AndNewConnection(t *testing.T) {
 					if err != nil {
 						t.Fatalf("socks5Server: proxy_test: Listen: %v", err)
 					}
-					defer func() {
-						close(closed)
-						l.Close()
-					}()
+					defer l.Close()
 
 					go func(shoulderror bool) {
 						conn, err := l.Accept()
@@ -584,6 +580,10 @@ func TestRoundTripSocks5AndNewConnection(t *testing.T) {
 						}
 
 						if err := proxyHandler.ServeConn(conn); err != nil && !shoulderror {
+							// If the connection request is closed before the channel is closed
+							// the test will fail with a ServeConn error. Since the test only return
+							// early if expects shouldError=true, the channel is closed at the end of
+							// the test, just before all the deferred connections Close() are executed.
 							if isClosed() {
 								return
 							}
@@ -684,6 +684,9 @@ func TestRoundTripSocks5AndNewConnection(t *testing.T) {
 							t.Fatalf("unexpected proxy user: %v", authUser)
 						}
 					}
+
+					// The channel must be closed before any of the connections are closed
+					close(closed)
 				})
 			}
 		})