Support seeding of Dialogs

/cc @gmcculloug 

Related BZ:
https://bugzilla.redhat.com/show_bug.cgi?id=1155806
Can be simplified to just `File.write(test_file_path, "test")`

In addition, I think it's weird that we are writing a test file into the real path.  Generally, we've been testing seeding by verifying that it's loading a real file, which admittedly is brittle if that file goes away, but it also tests real world seeding.  Usually I pick a file that most likely won't go away.

Another option is temporarily changing the seeding directory constant in the test with the `with_constants` helper, and then having it put to a known data directory under spec, like `spec/models/dialog/data`.