Improve pip package manager tests (#256)

Whilst writing the tests for the upcoming Poetry support, I made a few
changes to the overall package manager testing strategy (such as using
a testing buildpack to verify that at build time the tools and env vars
are configured correctly) - which I've split out of the later PRs for
easier review.

In particular, the new testing buildpack added here is what helped me
debug and locate this upstream lifecycle build time env vars bug (which
would have broken apps when we switch to venvs shortly):
buildpacks/lifecycle#1393

GUS-W-16617242.

tests/fixtures/pip_basic/requirements.txt

@@ -1,2 +1,2 @@
 # This package has been picked since it has no dependencies and is small/fast to install.
-typing-extensions==4.7.1
+typing-extensions==4.12.2

tests/fixtures/pip_editable_git_compiled/requirements.txt

@@ -8,4 +8,4 @@
 # The URL to the package is specified via env var, to test that user-provided env vars
 # are propagated to pip for use by its env var interpolation feature.
 
--e git+${WHEEL_PACKAGE_URL}@0.40.0#egg=extension.dist&subdirectory=tests/testdata/extension.dist
+-e git+${WHEEL_PACKAGE_URL}@0.44.0#egg=extension.dist&subdirectory=tests/testdata/extension.dist

tests/fixtures/testing_buildpack/bin/build

@@ -0,0 +1,24 @@
+#!/usr/bin/env bash
+
+# Check that:
+# - The correct env vars are set for later buildpacks.
+# - Python's sys.path is correct.
+# - The correct version of pip was installed.
+# - Both the package manager and Python can find the typing-extensions package.
+# - The system site-packages directory is protected against running 'pip install'
+#   without having passed '--user'.
+# - The typing-extensions package was installed into a separate dependencies layer.
+
+set -euo pipefail
+
+echo
+echo "## Testing buildpack ##"
+
+printenv | sort | grep -vE '^(_|CNB_.+|HOME|HOSTNAME|OLDPWD|PWD|SHLVL)='
+echo
+python -c 'import pprint, sys; pprint.pp(sys.path)'
+echo
+pip --version
+pip list
+pip install --dry-run typing-extensions
+python -c 'import typing_extensions; print(typing_extensions)'

tests/fixtures/testing_buildpack/bin/detect

@@ -0,0 +1,3 @@
+#!/usr/bin/env bash
+
+exit 0

tests/fixtures/testing_buildpack/buildpack.toml

@@ -0,0 +1,6 @@
+api = "0.11"
+
+[buildpack]
+id = "testing-buildpack"
+version = "0.0.0"
+clear-env = true

tests/mod.rs

@@ -3,16 +3,16 @@
 //! These tests are not run via automatic integration test discovery, but instead are
 //! imported in main.rs so that they have access to private APIs (see comment in main.rs).
 
-use libcnb_test::BuildConfig;
-use std::env;
-use std::path::Path;
-
 mod detect_test;
 mod django_test;
 mod package_manager_test;
 mod pip_test;
 mod python_version_test;
 
+use libcnb_test::BuildConfig;
+use std::env;
+use std::path::Path;
+
 const LATEST_PYTHON_3_7: &str = "3.7.17";
 const LATEST_PYTHON_3_8: &str = "3.8.19";
 const LATEST_PYTHON_3_9: &str = "3.9.19";