[27.x backport] opts: cleanup ParseEnvFile tests

[austinvazquez]

- What I did

• Backports opts: cleanup ParseEnvFile tests #5494 to 27.x

• 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

(cherry picked from commit b129660)

- How I did it

git cherry-pick -xsS b129660dd39f566c6cdc692467a99bb422001a98

- How to verify it

- Description for the changelog

n/a

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.78%. Comparing base (fbd51c0) to head (19279c9).
Report is 7 commits behind head on 27.x.

Additional details and impacted files

@@           Coverage Diff           @@
##             27.x    #5499   +/-   ##
=======================================
  Coverage   59.78%   59.78%           
=======================================
  Files         345      345           
  Lines       23442    23442           
=======================================
  Hits        14015    14015           
  Misses       8453     8453           
  Partials      974      974           

[thaJeztah]

LGTM, thanks!