test: add tests for automatic rehash after pip install

Using the brand new isolated_environment functionality in
test_helper.bash to set up a fresh and clean environment where we can
use pyenv to install a clean python version, and then check whether we
are getting an automatic rehash after pip install.

We are checking:

- pip
- pip3
- pip3.12
- python -m pip
- python3 -m pip
- python3.12 -m pip

for automatic rehash.

test/pip-rehash.bats

@@ -0,0 +1,132 @@
+#!/usr/bin/env bats
+
+# Test the automatic rehashing after doing a pip install.
+
+# Tell test_helper.bash to create an isolated environment.
+export ISOLATED_ENVIRONMENT=1
+load test_helper
+
+# Run once before all tests.
+# Sets up a fresh environment for testing.
+setup_file() {
+  eval "$(pyenv init -)"
+  assert_success
+
+  run pyenv install 3.12.8
+  assert_success
+
+  pyenv global 3.12.8
+  assert_success
+
+  # Add a dummy executable in case the computer running
+  # the tests has black installed in system python.
+  echo -e "#!/bin/bash\nexit 1" > "${PYENV_TEST_DIR}/bin/black"
+  chmod +x "${PYENV_TEST_DIR}/bin/black"
+}
+
+@test "auto rehash on pip install" {
+  # 1) Confirm that black is not found yet
+  run black --version
+  assert_failure
+  
+  # 2) Install black using pip
+  run pip install black
+  assert_success
+
+  # 3) Confirm that black is found after install (i.e. rehash happened)
+  run black --version
+  assert_success
+
+  # 4) Uninstall black using pip
+  run pip uninstall black -y
+  assert_success
+
+  # 5) Confirm that black is not found after uninstall
+  run black --version
+  assert_failure
+}
+
+@test "auto rehash on pip3 install" {
+  run black --version
+  assert_failure
+
+  run pip3 install black
+  assert_success
+
+  run black --version
+  assert_success
+
+  run pip3 uninstall black -y
+  assert_success
+
+  run black --version
+  assert_failure
+}
+
+@test "auto rehash on pip3.12 install" {
+  run black --version
+  assert_failure
+
+  run pip3.12 install black
+  assert_success
+
+  run black --version
+  assert_success
+
+  run pip3.12 uninstall black -y
+  assert_success
+
+  run black --version
+  assert_failure
+}
+
+@test "auto rehash on python -m pip install" {
+  run black --version
+  assert_failure
+
+  run python -m pip install black
+  assert_success
+
+  run black --version
+  assert_success
+
+  run python -m pip uninstall black -y
+  assert_success
+
+  run black --version
+  assert_failure
+}
+
+@test "auto rehash on python3 -m pip install" {
+  run black --version
+  assert_failure
+
+  run python3 -m pip install black
+  assert_success
+
+  run black --version
+  assert_success
+
+  run python3 -m pip uninstall black -y
+  assert_success
+
+  run black --version
+  assert_failure
+}
+
+@test "auto rehash on python3.12 -m pip install" {
+  run black --version
+  assert_failure
+
+  run python3.12 -m pip install black
+  assert_success
+
+  run black --version
+  assert_success
+
+  run python3.12 -m pip uninstall black -y
+  assert_success
+
+  run black --version
+  assert_failure
+}