TUN-6725: Fix testProxySSEAllData

This test was failing on Windows. We did not catch it before because our
TeamCity Windows builds were ignoring failed unit tests: TUN-6727

 - the fix is implementing WriteString for mockSSERespWriter
 - reason is because cfio.Copy was calling that, and not Write method,
   thus not triggering the usage of the channel for the test to continue
 - mockSSERespWriter was providing a valid implementation of WriteString
   via ResponseRecorder, which it implements via the embedded mockHTTPRespWriter
 - it is not clear why this only happened on Windows
 - changed it to be a top-level test since it did not share any code
   with other sub-tests in the same top-level test

proxy/proxy_test.go

@@ -130,6 +130,10 @@ func (w *mockSSERespWriter) Write(data []byte) (int, error) {
 	return len(data), nil
 }
 
+func (w *mockSSERespWriter) WriteString(str string) (int, error) {
+	return w.Write([]byte(str))
+}
+
 func (w *mockSSERespWriter) ReadBytes() []byte {
 	return <-w.writeNotification
 }
@@ -156,7 +160,6 @@ func TestProxySingleOrigin(t *testing.T) {
 	t.Run("testProxyHTTP", testProxyHTTP(proxy))
 	t.Run("testProxyWebsocket", testProxyWebsocket(proxy))
 	t.Run("testProxySSE", testProxySSE(proxy))
-	t.Run("testProxySSEAllData", testProxySSEAllData(proxy))
 	cancel()
 }
 
@@ -276,8 +279,7 @@ func testProxySSE(proxy connection.OriginProxy) func(t *testing.T) {
 
 // Regression test to guarantee that we always write the contents downstream even if EOF is reached without
 // hitting the delimiter
-func testProxySSEAllData(proxy *Proxy) func(t *testing.T) {
+func TestProxySSEAllData(t *testing.T) {
-	return func(t *testing.T) {
 	eyeballReader := io.NopCloser(strings.NewReader("data\r\r"))
 	responseWriter := newMockSSERespWriter()
 
@@ -286,7 +288,6 @@ func testProxySSEAllData(proxy *Proxy) func(t *testing.T) {
 
 	result := string(<-responseWriter.writeNotification)
 	require.Equal(t, "data\r\r", result)
-	}
 }
 
 func TestProxyMultipleOrigins(t *testing.T) {