Modify datamodels test to not check for ASDF tree in ImageHDU

[jdavies-st]

The storage of the ASDF tree in either an ImageHDU or another type of HDU is an ASDF implementation detail, and should not be tested here. This allows for the proposed change in ASDF PR 412 to put the ASDF tree in a NonstandardExtHDU instead of an ImageHDU.

[jdavies-st]

One further change in this PR is adding the .eggs directory in the repository to be ignored when running test. There's a corner case if you first run python setup.py test without having the test dependencies installed, it will install them in the .eggs/ subdir. Then if you later try to run pytest in the top level directory, it will try to run the tests inside the .eggs/ subdir, i.e. the tests in the package dependencies.

[hbushouse]

👍