Find a reliable way of balancing test load

[joakimk]

The only balancing done now is based on file-size which isn't very accurate.

A good solution would:

• Let testbot learn the test suite without any special benchmarking runs.
• Be able to balance based on how fast the various computers are.
• Have one job for each computer (reduce startup time overhead) and have the jobs end at the same time on all of them.
• Not be dependent on test framework native tools (such as rspec profiling)