Add a failing test for M2M models in test_merge_move_detail

[YPCrumble]

@saxix thanks for building this repo!

I'm trying to debug an issue when I try to merge a couple models with M2M relationships and attempting to create a failing test highlighting the issue.

The main problem is that I'm getting "This field is required" for various M2M fields in the Django admin when I submit the merge.

The first thing I tried to do is simply add a M2M relationship to the test_merge_move_detail test that seems to fail per this PR.

Should this test be expected to fail in the manner I've adapted it? I would have thought that the M2M relationship would be merged into the preserved model in this test.

Thanks for taking a look!