[lit, tests] Fix failing lit test: shtest-format.py

Summary:
The problem here is that on windows double quotes are used for paths (usually) while single quotes are not. This is not generally a problem for the tests because the lit infrastructure tends to treat both the same. One (and possibly only) exception is when some tests are run in an external shell such as some of the shtest-format tests. In this case on windows the path to python was not created correctly because it had single quotes and the test failed.

This same test is already failing with python 3 which is why our testing missed the new failure. This patch will take care of the immediate failure with python 2 and I'll send a follow up for the python 3 failure.

Reviewers: asmith, zturner

Subscribers: delcypher, llvm-commits

Differential Revision: https://reviews.llvm.org/D50373

llvm-svn: 339091