opts: cleanup ParseEnvFile tests

[thaJeztah]

• relates to introduce ParseKeyValueFile #5491

---

• 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)