[lit] Hack lit to allow a test suite to request that it is run "early".

This lets us for example start running the unit test suite early. For
'check-llvm' on my machine, this drops the tim e from 44s to 32s!!!!!

It's pretty ugly. I barely know how to write Python, so feel free to
just tell me how I should write it instead. =D Thanks to Filipe and
others for help.

Differential Revision: http://reviews.llvm.org/D18089

llvm-svn: 263329