Port TestBatchMode to PExpectTest class

Summary:
I'm doing this mainly for consistency, but there are also other cleanups
that will be enabled by this (e.g., the automatic setting of
clang-modules-cache-path setting).

Reviewers: teemperor, JDevlieghere

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D67082

llvm-svn: 370858