Add test for asdf data duplication issue with fits

[braingram]

See asdf issue asdf-format/asdf#1232

To summarize the issue, resaving an asdf in fits file breaks the link between hdu
items and arrays within the asdf tree. This results in inclusion of array data as
internal blocks in the asdf tree and a (sometimes substantial) increase in the size
of the tree. The test added here checks that the asdf tree stored in the 'ASDF' hdu
does not change size on a resave.

This test is currently skipped as it will fail with the current asdf version.

Checklist

• added entry in CHANGES.rst (either in Bug Fixes or Changes to API)
• updated relevant tests
• updated relevant documentation
• updated relevant milestone(s)
• added relevant label(s)

[nden]

Closed and opened to restart the tests

[codecov]

Codecov Report

Base: 83.78% // Head: 83.78% // No change to project coverage 👍

Coverage data is based on head (1d558fb) compared to base (f934400).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #105   +/-   ##
=======================================
  Coverage   83.78%   83.78%           
=======================================
  Files          10       10           
  Lines        1714     1714           
=======================================
  Hits         1436     1436           
  Misses        278      278           

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