Add test coverage to ensure that empty entity form does not appear with simple widget when not expected.

@skyredwang was reporting here that he was seeing an extra empty entity form on edit which stopped validation from working.

I can't reproduce this now but I did see this when I was writing the patch for the issue #2664626: Add another item does NOT work the first time it is clicked on simple widget

This issue will add test coverage for 3 things

1. On parent add when clicking "add another" on the simple widget only 1 new entity form shows up
2. On parent edit when clicking "add another" on the simple widget only 1 new entity form shows up
3. When editing entity with a simple widget no extra entity form is added until you click add another.

These tests should all pass now but if we need to tweak InlineEntityFormSimple::formMultipleElements these problems could arise.

I would rather get these test in now while somebody remembers this could happen.