[WIP] General cleanup for test_activate.py

[kenodegard]

Description

This should be one of the last PRs to be split from #13620. Included here are minor edits:

• removes unused monkeypatch fixtures
• uses arg unpacking instead of addition
• replace tempdir with tmp_path
• consolidates builder assertions to ensure no untested keys slip through, e.g.:

   # converts this:
   builder = activator.build_activate(prefix)
   ...
   assert builder["unset_vars"] == ...
   assert builder["set_vars"] == ...
   assert builder["export_vars"] == ...
   assert builder["activate_scripts"] == ...
   assert builder["deactivate_scripts"] == ...
   
   # into this:
   assert activator.build_activate(prefix) == {
       # "export_path": {},
       "deactivate_scripts": ...,
       "unset_vars": ...,
       "set_vars": ...,
       "export_vars": ...,
       "activate_scripts": ...,
   }

Extends #13760

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[codspeed-hq]

CodSpeed Performance Report

Merging #13763 will not alter performance

_{Comparing kenodegard:test_activate-cleanup (0cd2676) with main (298ca49)}

Summary

✅ 21 untouched benchmarks