TEST: Clear registry consistently to avoid order dependency

[effigies]

With xdist in particular, tests having implicit or explicit order dependencies can cause problems. The main one for sdcflows is the fieldmap registry global state.

This PR auto-clears before and after every doctest and adds a fixture for test_fieldmaps to auto-clear before and after every test in that file. I think we should probably do it globally.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.66%. Comparing base (1350d57) to head (aee2c92).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #464   +/-   ##
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  Coverage   83.66%   83.66%           
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  Files          32       32           
  Lines        2816     2816           
  Branches      377      377           
=======================================
  Hits         2356     2356           
  Misses        390      390           
  Partials       70       70           

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