opts: cleanup ParseEnvFile tests

[thaJeztah]

• relates to introduce ParseKeyValueFile #5491

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• Use gotest.tools for assertions
• Check for expected error messages
• Don't check for ErrBadKey errors, as it's not used as a sentinel error anywhere.
• Use t.SetEnv() instead of depending on HOME being set
• Use t.TempDir() for writing temporary files

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.10%. Comparing base (30e9abb) to head (b129660).
Report is 6 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5494   +/-   ##
=======================================
  Coverage   60.10%   60.10%           
=======================================
  Files         345      345           
  Lines       23447    23447           
=======================================
  Hits        14092    14092           
  Misses       8381     8381           
  Partials      974      974           

[thaJeztah]

Thx! I'll bring this one in; I was planning to deprecate the ErrBadKey error, so thought I'd first cleanup some of this (as it was checking for it)