tests: add guard check for suid support...
tests: add guard check for suid support
This adds a check for setuid and setgid support and guards a part of a test
requiring this particular filesystem feature.
Such feature is notably missing in the Nix/NixOS build sandbox and was causing
the whole test to fail for the mercurial package.
Differential Revision:
https://phab.mercurial-scm.org/D11739
