CurlHandler is deadlocking like WinHttpHandler (#28587)

* CurlHandler is deadlocking like WinHttpHandler

This repros consistently for Release on High Sierra. Issue #27746 already tracks it.

* Adding a comment to clarify scope

Author: Paulo Janotti

src/System.Net.Http/tests/FunctionalTests/HttpClientHandlerTest.cs

@@ -2940,9 +2940,11 @@ public async Task Proxy_SslProxyUnsupported_Throws()
         [Fact]
         public async Task Proxy_UseSecureProxyTunnel_Success()
         {
-            if (IsWinHttpHandler || IsNetfxHandler)
+            if (IsWinHttpHandler || IsNetfxHandler || IsCurlHandler)
             {
-                // Issue #27746: WinHttpHandler and netfx hang on this test
+                // Issue #27746: WinHttpHandler and netfx hang on this test. 
+                // The same happens consistently on macOS 10.13 Release and with some
+                // frequency on some Linux flavors, disabling the test for curl handler due to that.
                 return;
             }