Disable PreEnumerateTheories in default TestAssemblyConfiguration for…

… iOS/Android (#676)

It's already disabled for Browser and according to xunit/xunit#1746 (comment) the theory pre-enumeration only makes sense for UI scenarios like VS test explorer, which doesn't apply for xharness.

src/Microsoft.DotNet.XHarness.TestRunners.Xunit/XUnitTestRunner.cs

@@ -1081,7 +1081,7 @@ private async Task<XElement> Run(Assembly assembly, string assemblyPath)
             {
                 using (var discoverySink = new TestDiscoverySink())
                 {
-                    var configuration = GetConfiguration(assembly) ?? new TestAssemblyConfiguration();
+                    var configuration = GetConfiguration(assembly) ?? new TestAssemblyConfiguration() { PreEnumerateTheories = false };
                     ITestFrameworkDiscoveryOptions discoveryOptions = GetFrameworkOptionsForDiscovery(configuration);
                     discoveryOptions.SetSynchronousMessageReporting(true);
                     Logger.OnDebug($"Starting test discovery in the '{assembly}' assembly");