Move PreviewTests into their own scheme. (#2492)

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

PreviewTests/SupportingFiles/target.yml

@@ -0,0 +1,51 @@
+name: PreviewTests
+
+schemes:
+  PreviewTests:
+    analyze:
+      config: Debug
+    archive:
+      config: Release
+    build:
+      targets:
+        PreviewTests: all
+    profile:
+      config: Release
+    run:
+      config: Debug
+      disableMainThreadChecker: false
+    test:
+      config: Debug
+      disableMainThreadChecker: false
+      gatherCoverageData: true
+      coverageTargets:
+        - ElementX
+      testPlans:
+        - path: PreviewTests.xctestplan
+          defaultPlan: true
+
+targets:
+  PreviewTests:
+    type: bundle.unit-test
+    platform: iOS
+    
+    buildToolPlugins:
+    - plugin: PrefireTestsPlugin
+      package: Prefire
+
+    dependencies:
+    - target: ElementX
+    - package: SnapshotTesting
+
+    info:
+      path: ../SupportingFiles/Info.plist
+
+    settings:
+      base:
+        PRODUCT_NAME: PreviewTests
+        PRODUCT_BUNDLE_IDENTIFIER: ${BASE_BUNDLE_IDENTIFIER}.preview.tests
+      debug:
+      release:
+
+    sources:
+    - path: ../SupportingFiles