gh-129061: Fix FORCE_COLOR and NO_COLOR when empty strings

[hugovk]

First refactor the tests into subtests, which makes it easier compare the env var inputs and expected outputs.

• Issue: Empty values of FORCE_COLOR and NO_COLOR are not ignored #129061

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @hugovk for commit ec36e22 🤖

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https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F30617%2Fmerge

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[hugovk]

Like #129137 (comment), test_external_inspection failed on Fedora and RHEL8 buildbots and is unrelated.