Expand compare_asdf tests

[braingram]

compare_asdf is used extensively in the regression tests to compare truth and result files. It is minimally tested (see #868) Expanding these tests to include possibly the following would add assurance that this function is comparing the files as expected:

• compare two files that have a difference that should be ignored, verify that the difference is ignored
• compare two files with arrays that differ by a value less than the tolerance, verify that no difference is reported
• compare two files with wcs objects that differ, verify that this difference is detected