dotest.py - debug feature that helps find dosep races

SUMMARY
dosep.py starts lots and lots of dotest instances.

This option helps you find if two (or more) dotest instances are using
the same directory at the same time.

Enable it to cause test failures and stderr messages if dotest
instances try to run in the same directory simultaneously.

It is disabled by default because it litters the test directories with
".dirlock" files

TEST PLAN
Set lldbtest.debug_confirm_directory_exclusivity = True
run ./dosep.py

Differential Revision: http://reviews.llvm.org/D9868

llvm-svn: 237935