Fix the RedirectHandler (in Mvc.Testing) for relative URIs (dotnet#41742)

This was a case of double bugs cancelling each other. The `Testing/RedirectHandler/Redirect303` route was redirecting to `Testing/Builder` instead of `/Testing/Builder`, hiding the fact that the relative URL in the RedirectHandler was not handled properly.

A new test covering relative URLs has been added.

Author: 0xced

src/Mvc/Mvc.Testing/src/Handlers/RedirectHandler.cs

@@ -144,9 +144,7 @@ private static void UpdateRedirectRequest(
         {
             if (!location.IsAbsoluteUri && response.RequestMessage.RequestUri is Uri requestUri)
             {
-                location = new Uri(
-                    new Uri(requestUri.GetLeftPart(UriPartial.Authority)),
-                    location);
+                location = new Uri(requestUri, location);
             }
 
             redirect.RequestUri = location;

src/Mvc/test/Mvc.FunctionalTests/TestingInfrastructureTests.cs

@@ -109,6 +109,19 @@ public async Task TestingInfrastructure_RedirectHandlerUsesOriginalRequestHeader
         Assert.Equal("false", modifiedHeaderWasSent);
     }
 
+    [Fact]
+    public async Task TestingInfrastructure_RedirectHandlerHandlesRelativeLocation()
+    {
+        // Act
+        var request = new HttpRequestMessage(HttpMethod.Get, "Testing/RedirectHandler/Relative/");
+        var client = Factory.CreateDefaultClient(
+            new RedirectHandler());
+        var response = await client.SendAsync(request);
+
+        // Assert
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+    }
+
     [Fact]
     public async Task TestingInfrastructure_RedirectHandlerFollowsStatusCode303()
     {

src/Mvc/test/WebSites/BasicWebSite/Controllers/TestingController.cs

@@ -77,10 +77,19 @@ public IActionResult RedirectHandlerHeadersRedirect()
         }
     }
 
+    [HttpGet("Testing/RedirectHandler/Relative/")]
+    public IActionResult RedirectHandlerRelative()
+    {
+        return Redirect("Ok");
+    }
+
+    [HttpGet("Testing/RedirectHandler/Relative/Ok")]
+    public IActionResult RedirectHandlerRelativeOk() => Ok();
+
     [HttpGet("Testing/RedirectHandler/Redirect303")]
     public IActionResult RedirectHandlerStatusCode303()
     {
-        return new RedirectUsingStatusCode("Testing/Builder", HttpStatusCode.SeeOther);
+        return new RedirectUsingStatusCode("/Testing/Builder", HttpStatusCode.SeeOther);
     }
 
     public class RedirectUsingStatusCode : ActionResult

src/Mvc/test/WebSites/GenericHostWebSite/Controllers/TestingController.cs

@@ -63,10 +63,19 @@ public IActionResult RedirectHandlerHeadersRedirect()
         }
     }
 
+    [HttpGet("Testing/RedirectHandler/Relative/")]
+    public IActionResult RedirectHandlerRelative()
+    {
+        return Redirect("Ok");
+    }
+
+    [HttpGet("Testing/RedirectHandler/Relative/Ok")]
+    public IActionResult RedirectHandlerRelativeOk() => Ok();
+
     [HttpGet("Testing/RedirectHandler/Redirect303")]
     public IActionResult RedirectHandlerStatusCode303()
     {
-        return new RedirectUsingStatusCode("Testing/Builder", HttpStatusCode.SeeOther);
+        return new RedirectUsingStatusCode("/Testing/Builder", HttpStatusCode.SeeOther);
     }
 
     public class RedirectUsingStatusCode : ActionResult