To make the test runner work under msys-native Python...

Commit 5258566e broke the msys testsuite
driver (#10441). It changed the quoting of `config.compiler` from single
quotes to double quote, which turns out to not be compatible with what
the function `passThroughCmd` expected.

We could fix `passThroughCmd` to handle the case where `config.compiler`
is double quoted, and scatter some notes around to make sure the quoting
done in various places of the testsuite driver stay compatible.

Instead, this commit reverts 101c62e2,
which introdced the function `passThroughCmd` in the first place
(#9626). ezyang reports that doing this revert fixes the testsuite
driver for him using the the following version of msys2:

  msys2-keyring r8.3864337-1
  msys2-runtime 2.1.0.16351.cd3184b-1
  msys2-runtime-devel 2.1.0.16351.cd3184b-1
  msys2-w32api-headers 5.0.0.4456.c8b6742-1
  msys2-w32api-runtime 5.0.0.4455.32db221-1

Ideally we'd know what minimum version of msys2 we require, but for now
this fix is better than nothing.

Only gintas ever reported the original problem, and he actually
mentioned shortly afterwards: "This may have been fixed by a recent
release of msys2, but I am not sure."

Differential Revision: https://phabricator.haskell.org/D952

(cherry picked from commit bb996712)