Make lit TestRunner.py work in Python 3

Summary: In Python3 SubstituteCaptures are no longer converted to String implicitly behind the scenes. Converting explicitly makes the TestRunner to work in Python3.

Reviewers: gribozavr2, compnerd

Reviewed By: gribozavr2

Subscribers: tbkka, delcypher, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81361