Fix creation of test cache for clean checkout

[potiuk]

• I have added an entry to docs/changelog.md

(I think I do not need to add this change to changelog - it's purely development environment fix).

Summary of changes

The development environment test cache creation is pretty cool, but for first-time user creating the cache does not work cleanly because it misses directories and cache cannot be created.

Adding parents=True fixes the problem.

Test plan

Tested by running

rm -rf .pipx_tests .pytest_cache
pytest tests/test_install.py

This failed with:

Exception: Directory /Users/jarek/IdeaProjects/pipx/.pipx_tests/package_cache does not contain all package distribution files necessary to run pipx tests. Please run the following command to populate it: /Users/jarek/IdeaProjects/pipx/venv/bin/python scripts/update_package_cache.py testdata/tests_packages /Users/jarek/IdeaProjects/pipx/.pipx_tests/package_cache

Before the fix running:

/Users/jarek/IdeaProjects/pipx/venv/bin/python scripts/update_package_cache.py testdata/tests_packages /Users/jarek/IdeaProjects/pipx/.pipx_tests/package_cache

causes "No such file or directory"

After the fix, environment gets setup properly