test/upgrade: Test multiple upgrade path in Ubuntu

Originally, we tested only precise->saucy, through non-LTS releases, but this got broken when we marked raring as unsupported.

Now, we test quantal->saucy->trusty (this skips raring, as it was EOL before quantal), raring->saucy, and LTS upgrade precise->trusty.

precise->quantal is broken (upstream problem due to, at least, `libxi6` needing to be downgraded), but I don't see why anyone would still want to do that now that trusty has been released.

Finally, the test only runs if `-R` is not specified on the command line: we do not want to check against `$SUPPORTED_RELEASES` as this test intentionally tries out releases that are marked as unsupported.

Tested in `2014-04-20_11-38-30_drinkcat_chroagh_upgrade-test_18` (nothing broke!)