[3.10] gh-93975: Provide nicer error reporting from subprocesses in test_venv.EnsureP...

[3.10] gh-93975: Provide nicer error reporting from subprocesses in test_venv.EnsurePipTest.test_with_pip (GH-93959) (GH-94004)

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.
(cherry picked from commit 6066f450b91f1cbebf33a245c14e660052ccd90a)

Co-authored-by: Jason R. Coombs <jaraco@jaraco.com>