contrib/net/http: fix flaky TestRoundTripperNetworkError (#1292)

The test was intended to cause an HTTP request to time out on the client
side, but this timeout was induced with a short sleep in the HTTP
handler. This caused the test to sometimes fail in CI. Removing the
sleep made the test fail consistently.

To force the client to time out, make the handler wait on a channel that
doesn't close until the end of the test.

This fix can be verified locally by setting the client timeout to a very
long value like 1 minute. The test will still pass.

Fixes #1291.

contrib/net/http/roundtripper_test.go

@@ -123,13 +123,14 @@ func TestRoundTripperNetworkError(t *testing.T) {
 	mt := mocktracer.Start()
 	defer mt.Stop()
 
+	done := make(chan struct{})
 	s := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		_, err := tracer.Extract(tracer.HTTPHeadersCarrier(r.Header))
 		assert.NoError(t, err)
-		time.Sleep(10 * time.Millisecond)
-		w.Write([]byte("Timeout"))
+		<-done
 	}))
 	defer s.Close()
+	defer close(done)
 
 	rt := WrapRoundTripper(http.DefaultTransport,
 		WithBefore(func(req *http.Request, span ddtrace.Span) {