don't run if insufficient free space on cassandane rootdir partition

[elliefm]

It'd be nice if testrunner.pl could check the available space on the "rootdir" partition, and not run if there's not enough space for it to finish.

At the moment it just runs, and if the disk fills during the run, you get a tonne of terminal spam from all the tests that failed due to the filled disk.

Probably do a run in a clean rootdir to see how much space a single run consumes, double+fudge and use that as a threshold?