Improve tests some more
Created by: mkhl
these changes:
- improve output when a test fails
- unset XDG_CURRENT_DESKTOP in most tests so we don't accidentally assume it's defined
- add tests for missing directories
- add a test for the xterm fallback
- add a test for quoting arguments, both in the Exec line and to xdg-terminal-exec
the quoting test currently fails and i can remove it if you prefer