test_custom_and_analytical leaves open file in cwd #9705

Description

After merging #9522 the new test fails on open_files=True and leaves the open file custom_and_analytical_inverse.asdf, which is created in cwd instead of tmpdir, behind.
See review and comment for details.

Expected behavior

Should use something like

fa.write_to(str(tmpdir.join('custom_and_analytical_inverse.asdf')))

and close the file on return.

System Details

OS X 10.12.6
Python 3.6.9, 3.7.5, 3.8.0
Numpy: 1.17.4
Scipy: 1.3.2
Astropy 4.0rc1, 4.1.dev27006