Wait after logout() in account_settings tests (bug 1020262)

That wasn't necessary before, but in e5aa1399 we introduced a new
API call for logout, so it's not done instantaneously.
Wait till we see the buttons that show up when logged out (bug 1020262)
Check system date when starting packaged Marketplace (bug 1019122)

If the device clock is too far behind the user will get a certificate
error, so let's display our own error message instead.