Bug 72903 Add variable expansion for Test Class in JUnit launch configurations [JUnit]
org.eclipse.jdt.junit/src/org/eclipse/jdt/internal/junit/launcher/JUnitMainTab.java
private void testModeChanged() {
		boolean isSingleTestMode= fTestRadioButton.getSelection();
		setEnableSingleTestGroup(isSingleTestMode);
		setEnableContainerTestGroup(!isSingleTestMode);
		if (!isSingleTestMode && fContainerText.getText().length() == 0) {
			IJavaProject javaProject= getJavaModel().getJavaProject(fProjText.getText());
			if (javaProject != null && javaProject.exists())
				setContainerElement(javaProject);
		}
		validatePage();
		updateLaunchConfigurationDialog();
	}
	private void validatePage() {
		setErrorMessage(null);
		setMessage(null);
		
		if (fTestContainerRadioButton.getSelection()) {
			if (fContainerElement == null)
				setErrorMessage(JUnitMessages.getString("JUnitMainTab.error.noContainer")); 
			return;
		} 
		
		String projectName = fProjText.getText().trim();
		if (projectName.length() == 0) {
			setErrorMessage(JUnitMessages.getString("JUnitMainTab.error.projectnotdefined")); 
			return;
		}
			
		IProject project = getWorkspaceRoot().getProject(projectName);
		if (!project.exists()) {