EACCES during test_mount on Cirrus-CI

[asomers]

trafficstars

test_mount fails when run on Cirrus-CI. The mount syscall returns EACCES. According to the man page, there are only three reasons for that error, and none of them apply in this case. For now we'll skip the test on Cirrus, but we need to find the root cause.

This test was always skipped on Travis, because unshare would fail with EPERM, indicating that unprivileged user namespaces are not available in that environment.