Fix GN unittests on windows broken by abs-path change

Mixed up gen_dir and out_dir in one win-only unit test in a recent CL,
https://codereview.chromium.org/1420973003

R=brettw@chromium.org, andybons@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1408793007

Cr-Original-Commit-Position: refs/heads/master@{#356945}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 784ffb68d74d58986d58aaa88e33573608f548ee