tests: add coverage for --require-virtualenv

[LouisLau-art]

Fixes #12843.

Adds unit tests covering:

• --require-virtualenv exits with VIRTUALENV_NOT_FOUND when not running under a virtualenv.
• Commands that opt out via ignore_require_venv = True (e.g. pip help) continue to work.
• An explicit matrix test that enumerates pip commands and asserts whether each opts in/out of --require-virtualenv.

Tests:

• python -m pytest tests/unit/test_base_command.py -q

[notatallshaw]

While I appreciate testing PRs, especially unit tests, I don't think this fixes #12843 which notes that none of the functionality is tested, and this doesn't test any of the functionality. Would you agree?