gh-93975: Nicer error reporting in test_venv (GH-93959)

- gh-93957: Provide nicer error reporting from subprocesses in test_venv.EnsurePipTest.test_with_pip.
- Update changelog

This change does three things:

1. Extract a function for trapping output in subprocesses.
2. Emit both stdout and stderr when encountering an error.
3. Apply the change to `ensurepip._uninstall` check.