[nextest-runner] add OutputSpec for output types

[sunshowers]

For upcoming log file support we're going to need a different way to represent live outputs. Prepare for that -- this is good code hygiene anyway.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.27%. Comparing base (c49d388) to head (951a67c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3089   +/-   ##
=======================================
  Coverage   84.27%   84.27%           
=======================================
  Files         156      156           
  Lines       42455    42446    -9     
=======================================
- Hits        35777    35772    -5     
+ Misses       6678     6674    -4     

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[trunk-io]

     

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