Replace custom AssertionFailedErrors in test harness based on org.junit

The custom AssertionFailedErrors specified in
org.eclipse.core.tests.harness are derived from JUnit 3/4's
AssertionFailedError coming from org.junit. In order to fully migrate
the test harness to JUnit 5, this change adapts those classes to
subclass the AssertionFailedError from org.opentest4j as done by JUnit 5
as well.

Contributes to
#903

Author: HeikoKlare

runtime/tests/org.eclipse.core.tests.harness/src/org/eclipse/core/tests/harness/FussyProgressMonitor.java

@@ -15,8 +15,8 @@
 
 import static org.assertj.core.api.Assertions.assertThat;
 
-import junit.framework.AssertionFailedError;
 import org.eclipse.core.runtime.jobs.Job;
+import org.opentest4j.AssertionFailedError;
 
 /**
  * This class can be used for testing progress monitoring.

runtime/tests/org.eclipse.core.tests.harness/src/org/eclipse/core/tests/session/RemoteAssertionFailedError.java

@@ -15,7 +15,7 @@
 
 import java.io.PrintStream;
 import java.io.PrintWriter;
-import junit.framework.AssertionFailedError;
+import org.opentest4j.AssertionFailedError;
 
 public class RemoteAssertionFailedError extends AssertionFailedError {
 	/**