Move tests with external servers to outerloop (dotnet/corefx#37934)

* move tests with external servers to outerloop

* fix Set_ValidValues_Success


Commit migrated from dotnet/corefx@d58a8ad

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

@@ -51,15 +51,7 @@ public void Set_ValidValues_Success(int validValue)
         {
             using (HttpClientHandler handler = CreateHttpClientHandler())
             {
-                try
-                {
-                    handler.MaxConnectionsPerServer = validValue;
-                }
-                catch (PlatformNotSupportedException)
-                {
-                    // Some older libcurls used in some of our Linux CI systems don't support this
-                    Assert.True(RuntimeInformation.IsOSPlatform(OSPlatform.Linux));
-                }
+                handler.MaxConnectionsPerServer = validValue;
             }
         }
 
@@ -71,6 +63,7 @@ public void Set_ValidValues_Success(int validValue)
         [InlineData(3, 2, false)]
         [InlineData(3, 2, true)]
         [InlineData(3, 5, false)]
+        [OuterLoop("Uses external servers")]
         public async Task GetAsync_MaxLimited_ConcurrentCallsStillSucceed(int maxConnections, int numRequests, bool secure)
         {
             using (HttpClientHandler handler = CreateHttpClientHandler())