Add an off-by-default option to enable testing for gdb pretty printers.

Summary:
The current version of the pretty printers are not python3 compatible,
so turn them off by default until sufficiently improved.

Reviewers: MaskRay, tamur

Subscribers: mgorny, christof, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68477

llvm-svn: 373796