Fix dotest.py '-p' option for multi-process mode

Summary:
The '-p' option for dotest.py was ignored in multiprocess mode,
as the -p argument to the inferior would overwrite the -p argument
passed on the command line.

Reviewers: zturner, tfiala

Subscribers: lldb-commits, sas

Differential Revision: http://reviews.llvm.org/D18779

Change by Francis Ricci <fjricci@fb.com>

This is a cherry-pick of r265422

llvm-svn: 270129