Fix TAP syntax in test-basic-user.sh, and run it

Fix TAP syntax in test-basic-user.sh, and run it

In its initial commit, Alexander Larsson wrote

    This works standalone, but unfortunately it breaks in
    gnome-desktop-testing-runner as /tmp doesn't support
    xattrs, so it is not installed atm.

but we now (a) use /var/tmp, and (b) explicitly skip the test if
xattr support is unavailable. So it should be OK to run now.

Closes: #652
Approved by: cgwalters