Update testing paths and commands

[forsyth2]

Summary

Objectives:

• Make sure all environment_commands variants get set to use Unified when doing unified testing
• Allow distinct parent directories for v2 and v3 testing data.

Select one: This pull request is...

• a bug fix: increment the patch version
• a small improvement: increment the minor version
• a new feature: increment the minor version
• an incompatible (non-backwards compatible) API change: increment the major version

Small Change

• To merge, I will use "Squash and merge". That is, this change should be a single commit.
• Logic: I have visually inspected the entire pull request myself.
  • The primary changes are in tests/integration/utils.py. Everything else is a minor update to accommodate these changes or is auto-generated.
• Pre-commit checks: All the pre-commits checks have passed.

[forsyth2]

Might be useful to wrap this in a if expansions["diags_environment_commands"] == "":, so it doesn't overwrite an explicitly set diags_environment_commands. Currently testing with Unified only, so fine for now.

[forsyth2]

Addressed at #676