gradle: Refactor and update to gradle 6

.github/workflows/pr.yml

@@ -60,8 +60,8 @@ jobs:
       - name: Testing
         run: ./gradlew test --stacktrace --console=plain
 
-  checkstyle:
-    name: Checkstyle
+  checkstyleMain:
+    name: Checkstyle main
     runs-on: ubuntu-latest
 
     steps:
@@ -83,9 +83,32 @@ jobs:
       - name: Checking code conventions
         run: ./gradlew checkstyleMain --console=plain
 
+  checkstyleTest:
+    name: Checkstyle test
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v1
+      - uses: actions/cache@v1
+        with:
+          path: ~/.gradle/caches
+          key: ${{ runner.os }}-gradle-${{ hashFiles('**/*.gradle.kts') }}
+          restore-keys: |
+            ${{ runner.os }}-gradle-
+      - name: Make gradlew executable
+        run: chmod +x ./gradlew
+      - name: Set up JDK 11
+        uses: actions/setup-java@v1
+        with:
+          java-version: 11
+      - name: Assembling
+        run: ./gradlew assemble --console=plain
+      - name: Checking code conventions
+        run: ./gradlew checkstyleTest --console=plain
+
   approve:
     name: Approve
-    needs: [build, test, checkstyle]
+    needs: [build, test, checkstyleMain, checkstyleTest]
     runs-on: ubuntu-latest
 
     steps: