Tests - Disable GPU Acceleration

There have been transient crashes of the tests when run via Appveyor

https://ci.appveyor.com/project/cefsharp/cefsharp/builds/51568290#L3321

Author: amaitland

CefSharp.Test/CefSharpFixture.cs

@@ -58,6 +58,7 @@ private void CefInitialize()
                 settings.RootCachePath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData), "CefSharp\\Tests");
                 //settings.CefCommandLineArgs.Add("renderer-startup-dialog");
                 //settings.CefCommandLineArgs.Add("disable-site-isolation-trials");
+                settings.DisableGpuAcceleration();
 
                 var success = Cef.Initialize(settings, performDependencyCheck: false, browserProcessHandler: null);