Tests: create a copy of Unmigratable vault, then delete it after the test runs

[fmaclen]

We have a corrupt Unmigratable.vault we use to test that when a vault isn't migratable an error is displayed to the user.
But when the test runs it modifies this Unmigratable vault causing a Git diff.

These vaults need to remain in the repo (can't add them to.gitignore) so a solution could be to create temporary copies of the files, run the test, and then during the test cleanup delete such files.