tests: avoid false failures with non default groups

tests: avoid false failures with non default groups

On OS X it was seen that the group ID used for new files,
are set to a that of the directory rather than the current user.
It's not currently understood when this happens, but it was confirmed
that ACLs, extended attributes and setgid bits are _not_ involved.

* init.cfg (skip_if_nondefault_group_): A new function to detect
and avoid this situation.  Document with links to the discussions
for hopefully future clarification.
* tests/install/install-C-root.sh: Use the new function.
* tests/install/install-C-selinux.sh: Likewise.
* tests/install/install-C.sh: Likewise.