test-execute: skip flaky test when we can't unshare namespaces

test-execute: skip flaky test when we can't unshare namespaces

When running in Fedora "mock", / is a tmpfs and /home is not mounted. The test
assumes that /home will be a tmpfs only and only if we can unshare. Obviously,
this does not hold in this case, because unsharing is not possible, but /home
is still a tmpfs. Let's just skip the test, since it's fully legitimate to
mount either or both of / and /home as tmpfs.