setup testing on an nfs4_*acl cripped discovery partition

after #102 closing #57 is done, and we polish up support for freeze/thawing with external scripts (https://github.com/datalad/datalad/issues/6497 and references there in) -- we should have operation with use of such scripts be tested. I think it should be done by making the "test HOME" to have `~/.gitconfig` which would configure annex to use those scripts, and then running datalad tests with TMPDIR on that partition, while ensuring that we are not falling into adjusted branches unexpectedly (i think there might be need in tests changes)