Allow specifying eunit suite run order in 'suites'

Normally, Rebar runs eunit tests in the order the beam files are stored in the file system (see `rebar_utils:beams`).  However, sometimes it is desirable to run the tests in a different order (e.g. to reproduce an error found on a build server).  For that case, it would make sense to use the `suites` parameter not just for selecting which modules to consider, but also for choosing the order.