Add test for writing nexus files early

[DominicOram]

Fixes #677

To test:

1. Confirm tests fail after pip install nexgen==0.6.26
2. Confirm tests pass after pip install nexgen==0.6.27
3. Confirm commented issue will solve skipped test

[codecov]

Codecov Report

Merging #680 (481a308) into main (bc18cce) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #680   +/-   ##
=======================================
  Coverage   89.75%   89.75%           
=======================================
  Files          34       34           
  Lines        1494     1494           
=======================================
  Hits         1341     1341           
  Misses        153      153           

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[d-perl]

tests pass and fail as described, the skipped test previously worked for correct nexus files so that issue should fix it. thanks!