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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

```diff @@ 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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