doc:  improve configuration management section

Patch by David Johnston

Backpatch through 9.4
Support older versions of "prove"

Apparently, older versions of "prove" (couldn't identify the exact
version from the changelog) don't look into the t/ directory for tests
by default, so specify it explicitly.
Handle old versions of Test::More

Really old versions of Test::More don't support subplans, so skip the
tests in that case.