Set the plan when using xdist.

[mblayman]

Fixes #71

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (80f4fee) 100.00% compared to head (81db6cb) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #76   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           69        73    +4     
=========================================
+ Hits            69        73    +4     

Impacted Files	Coverage Δ
src/pytest_tap/plugin.py	100.00% <100.00%> (ø)

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[mblayman]

@allisonkarlitskaya, thank you! I shamelessly used your implementation as a very strong inspiration for this change. I've added you to the AUTHORS file in the same way you credited yourself in the tappy project.

This still doesn't solve your challenge of tap.py not being packaged in Fedora, but hopefully it's closer to what you need.