Move PreviewTests into their own scheme. (#2492)

* Use xcresultparser for coverage as there are 2 schemes now.

fastlane/Fastfile

@@ -88,15 +88,17 @@ lane :unit_tests do
     ensure_devices_found: true,
     result_bundle: true,
     number_of_retries: 3,
+  )
+  
+  run_tests(
+    scheme: "PreviewTests",
+    device: 'iPhone 14 (16.4)',
+    result_bundle: true,
+    number_of_retries: 3,
     xcargs: '-skipPackagePluginValidation',
   )
-
-  slather(
-    cobertura_xml: true,
-    output_directory: "./fastlane/test_output",
-    proj: "ElementX.xcodeproj",
-    scheme: "UnitTests",
-  )
+  
+  # We use xcresultparser in the workflow to collect coverage from both result bundles.
 end
 
 lane :ui_tests do |options|