TEST: Chdir during doctest to avoid polluting the working dir

[effigies]

Small cleanup. When running the test suite, simple.nc gets dropped into the working directory by the netcdf doctests. This detects a doctest in the externals directory and chdirs into a temporary directory to guard us against these, as we'd rather not modify the vendored files more than necessary.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6ba501e) 92.24% compared to head (33c21d4) 92.24%.
Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1279   +/-   ##
=======================================
  Coverage   92.24%   92.24%           
=======================================
  Files          99       99           
  Lines       12452    12452           
  Branches     2560     2560           
=======================================
  Hits        11486    11486           
  Misses        643      643           
  Partials      323      323           

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