test: expand pipenv locator coverage (#436)

Adds 19 new unit tests to pet-pipenv (from 5 to 24 total).

New coverage:
- get_pipenv_virtualenv_dirs(): WORKON_HOME, XDG_DATA_HOME,
~/.local/share/virtualenvs, ~/.venvs, ~/.virtualenvs, empty/non-existent
directories
- get_virtualenvs_dir(): WORKON_HOME preference, platform defaults, none
case
- is_pipenv_from_project(): prefix-based, exe-path derivation, missing
Pipfile
- is_pipenv(): plain venv rejection
- get_pipenv_project(): .project file reading, none case
- is_in_pipenv_centralized_dir(): prefix derivation from executable,
non-derivable prefix
- Locator metadata validation

Fixes #389

Author: karthiknadig