Prevent multiple execution of junit.runtime.tests and always clear WS

Fixes #1360

Author: HannesWell

ui/org.eclipse.pde.junit.runtime.tests/pom.xml

@@ -32,6 +32,7 @@
 				<groupId>org.eclipse.tycho</groupId>
 				<artifactId>tycho-surefire-plugin</artifactId>
 				<configuration>
+					<testClass>org.eclipse.pde.junit.runtime.tests.JUnitRuntimeTests</testClass>
 					<useUIHarness>true</useUIHarness>
 					<useUIThread>true</useUIThread>
 				</configuration>

ui/org.eclipse.pde.junit.runtime.tests/src/org/eclipse/pde/junit/runtime/tests/JUnit5SuiteExecutionTest.java

@@ -24,12 +24,18 @@
 import org.eclipse.jdt.junit.model.ITestElement;
 import org.eclipse.jdt.junit.model.ITestElementContainer;
 import org.eclipse.jdt.junit.model.ITestRunSession;
+import org.eclipse.pde.ui.tests.util.ProjectUtils;
 import org.junit.Assert;
 import org.junit.BeforeClass;
+import org.junit.ClassRule;
 import org.junit.Test;
+import org.junit.rules.TestRule;
 
 public class JUnit5SuiteExecutionTest {
 
+	@ClassRule
+	public static final TestRule CLEAR_WORKSPACE = ProjectUtils.DELETE_ALL_WORKSPACE_PROJECTS_BEFORE_AND_AFTER;
+
 	private static IJavaProject project;
 
 	@BeforeClass