Remove `DataModel` from alignment tests

[braingram]

The alignment tests include a DataModel class:

stcal/tests/test_alignment.py

Line 100 in eb8528c

class DataModel:

which doesn't match jwst or roman (and doesn't pass isinstance with the AbstractDataModel class in stpipe).

With the changes in #267 we may be able to remove DataModel Metadata, Coordinates and WcsInfo to simplify the alignment tests.