test/upgrade: Test multiple upgrade path in Ubuntu

Originally, we tested only precise->saucy, through non-LTS releases.

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.