[docs] TestingGuide.rst: Fix incorrect description

This patch fixes two incorrect descriptions in TestingGuide.rst.

1. test/lit.site.cfg --> test/lit.site.cfg.py
   After https://reviews.llvm.org/D37838 , the `test/lit.site.cfg` had been added a .py extension.
   So it should be `test/lit.site.cfg.py`.

2. $(LLVM_OBJ_ROOT)/$(BuildMode)/bin --> $(LLVM_OBJ_ROOT)/bin
   The current build system doesn't create a $(BuildMode) directory any more.
   So it should be removed.

Reviewed By: mehdi_amini, MaskRay

Differential Revision: https://reviews.llvm.org/D140780