[dotest] Use subprocess.call to forward arguments in wrapper

As suggested by Pavel on lldb-commits. Originally I picked os.system
because it was so much more simple than the subprocess module, but that
no longer holds true after yesterday's hack in r328020. This is what it
should've been in the first place.

Differential revision: https://reviews.llvm.org/D44728

llvm-svn: 328089