Scala 2.13 support. (#9099)

1. Updated the test logic
2. Added smoke tests for Spark examples.
3. Added integration tests for Spark with Scala 2.13

jvm-packages/xgboost4j-tester/src/test/java/ml/dmlc/xgboost4j/tester/AppTest.java

@@ -1,20 +0,0 @@
-package ml.dmlc.xgboost4j.tester;
-
-import static org.junit.Assert.assertTrue;
-
-import org.junit.Test;
-
-/**
- * Unit test for simple App.
- */
-public class AppTest 
-{
-    /**
-     * Rigorous Test :-)
-     */
-    @Test
-    public void shouldAnswerWithTrue()
-    {
-        assertTrue( true );
-    }
-}