Add bitfield Display regression tests

[macladson]

Adds a small regression test for bitfield's Display impl

Note that the blitlist uses a bit_len of 15 to check if trailing zeros are removed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.01%. Comparing base (7737481) to head (b40f8ca).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #60      +/-   ##
==========================================
+ Coverage   76.13%   77.01%   +0.87%     
==========================================
  Files          13       13              
  Lines         796      796              
==========================================
+ Hits          606      613       +7     
+ Misses        190      183       -7     

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