Revert "Adjust Gradle to JUnit 5"

This reverts commit 2ff8ac4.

Author: manticore-projects

src/test/java/net/sf/jsqlparser/statement/select/SpeedTest.java

@@ -23,10 +23,7 @@
 import net.sf.jsqlparser.test.TestException;
 import net.sf.jsqlparser.util.TablesNamesFinder;
 import org.junit.jupiter.api.Test;
-import org.junit.jupiter.api.parallel.Execution;
-import org.junit.jupiter.api.parallel.ExecutionMode;
 
-@Execution(ExecutionMode.CONCURRENT)
 public class SpeedTest {
 
     private final static int NUM_REPS_500 = 500;