Client-side tests

This spins up a Minecraft instance (much like we do for the server) and
instructs the client to take screenshots at particular times. We then
compare those screenshots and assert they match (modulo some small
delta).

.github/workflows/main-ci.yml

@@ -32,7 +32,7 @@ jobs:
       run: |
         ./gradlew assemble || ./gradlew assemble
         ./gradlew downloadAssets || ./gradlew downloadAssets
-        ./gradlew build
+        xvfb-run ./gradlew build
 
     - name: Upload Jar
       uses: actions/upload-artifact@v2
@@ -40,6 +40,15 @@ jobs:
         name: CC-Tweaked
         path: build/libs
 
+    - name: Upload Screnshots
+      uses: actions/upload-artifact@v2
+      with:
+        name: Screenshots
+        path: test-files/client/screenshots
+        if-no-files-found: ignore
+        retention-days: 5
+      if: failure()
+
     - name: Upload Coverage
       uses: codecov/codecov-action@v1