Support GoogleTest's "typed tests"

Support GoogleTest's "typed tests"
(http://code.google.com/p/googletest/wiki/GoogleTestAdvancedGuide#Typed_Tests)
in lit.py.  These tests have names like "ValueMapTest/0.Iteration", which broke
when lit.py os.path.join()ed them onto the path and then assumed it could
os.path.split() them back off.  This patch shifts path components from the
testPath to the testName until the testPath exists.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@84387 91177308-0d34-0410-b5e6-96231b3b80d8