trigger NoteOnContentDocument on ContentDocument (before delete) {
   for (ContentDocument c : Trigger.old){
     List<ContentDocumentLink> links = [SELECT LinkedEntityId FROM ContentDocumentLink
     WHERE ContentDocumentId= :c.Id];
    if (!links.isEmpty() && Approval.isLocked(links.get(0).LinkedEntityId)){
     c.addError('Approval pending. You do not have the permission to edit/delete this note/attachment, please contact your administrator.');
    }
  }
}

@istest
public class TestNoteOnContentDocuments {
  static testmethod void NoteOnContentDoc(){
    ContentVersion contentVersion = new ContentVersion(
      Title = 'Penguins',
      PathOnClient = 'Penguins.jpg',
      VersionData = Blob.valueOf('Test Content'),
      IsMajorVersion = true
    );
    insert contentVersion;
    List<ContentDocument> documents = [SELECT Id, Title, LatestPublishedVersionId FROM ContentDocument];
    delete documents;

     DmlException expectedException;
    Test.startTest();
        try { delete documents; }
        catch (DmlException dmx) {

       Boolean expectedExceptionThrown = dmx.getMessage().contains('Approval pending. You do not have the permission to edit/delete this note/attachment, please contact your administrator.') ? true : false;
       System.assertEquals(expectedExceptionThrown, true);
        }
    Test.stopTest();
 }
}