Make dosep correctly invoke the top-level script when forking out

packages/Python/lldbsuite is now a Python package, and it relies
on its __init__.py being called to do package-level initialization.
If you exec packages/Python/lldbsuite/dotest.py directly, you won't
get this package level initialization, and things will fail.  But
without this patch, this is exactly what dosep itself does.  To
launch the multi-processing fork, it was hardcoding a path to
dotest.py and exec'ing it from inside the package.

The fix here is to get the path of the top-level script, and
then exec'ing that instead.  A more robust solution would involve
refactoring the code so that dosep execs some internal script that
imports lldbsuite, but that's a bit more involved.

Differential Revision: http://reviews.llvm.org/D14157
Reviewed by: Todd Fiala

llvm-svn: 251819