testdirs: assert against real config/cache paths, not $HOME prefix

Review feedback on #1411: the fallback uses os.MkdirTemp, which respects TMPDIR — and TMPDIR can legitimately live under $HOME. The invariant is "never the real ~/.config/entire or ~/.cache/entire", not "outside the home directory". Tighten the two regression tests to assert exactly that, and reword the doc comments to stop implying the fallback is guaranteed to be outside $HOME.