Use venv for Python tests

[zmoon]

so that it will work in read-only container

[Copilot]

Pull request overview

This PR updates CECE’s CTest-based Python test setup to run inside a build-directory virtual environment so test dependencies can be installed even when the base environment is read-only (e.g., certain container runtimes). It also adjusts the GitHub Actions workflow to avoid installing some Python test deps globally since they’ll be installed into the venv by CTest fixtures.

Changes:

• Create a Python venv under the CMake build directory and run Python/pytest tests using that venv interpreter.
• Add CTest fixtures to upgrade pip and install Python test dependencies into the venv before running Python tests.
• Update CI to stop globally installing pytest/pyyaml (now installed by CTest fixtures) while keeping deps needed for scripts run outside CTest.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
CMakeLists.txt	Adds build-dir venv creation + fixture-driven pip installs; switches Python tests to use the venv interpreter.
.github/workflows/ci.yml	Removes global pytest/pyyaml install; documents that CTest fixtures handle these in the build-dir venv.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.