This should fix the testsuite driver on Windows using the MinGW tools
with a native build of Python.

MinGW automagically converts MinGW-style paths (e.g.
'/c/programs/ghc/bin/ghc') into ordinary Windows paths (e.g.
'C:/programs/ghc/bin/ghc') when a native Windows program is invoked. But
it doesn't do so when those paths are wrapped with a pair of escaped
double quotes.

The fix is to not call `eval` on the paths in Python, which let's us use
one less pair of quotes, and makes MinGW happy.

Reviewers: Rufflewind, austin

Differential Revision: https://phabricator.haskell.org/D911

(cherry picked from commit ce166a3a)