Help in writing unit tests

Hello,

I'm new to plugin development, and I'm looking for help in writing unit tests for actions & activities.

For example, how would I write a unit test for this very simple action (a file picker and message dialog):

public class FileAccessAction extends AnAction {
    @Override
    public void actionPerformed(@NotNull AnActionEvent e) {
        VirtualFile vFile = getFile(e.getProject());
        if (vFile != null) {
            Messages.showMessageDialog(e.getProject(), "Success", "File Selection", Messages.getInformationIcon());
        } else {
           Messages.showMessageDialog(e.getProject(), "Fail", "File Selection", Messages.getErrorIcon());
        }
    }

    private VirtualFile getFile(Project project) {
        FileChooserDescriptor fileChooser = new FileChooserDescriptor(true, false, false, false, false, false);
        fileChooser.setTitle("Select File");
        fileChooser.setDescription("Select the file");
        return FileChooser.chooseFile(fileChooser, project, null);
   }

}

Do I need to use mocks? If yes, can you please give an example.