[GCOV] fix test after patch rL346642

Summary:
Test is failing under windows, so fix it.
Should fix:
http://lab.llvm.org:8011/builders/clang-x64-windows-msvc/builds/1390/steps/stage%201%20check/logs/stdio

Reviewers: marco-c

Reviewed By: marco-c

Subscribers: cfe-commits, sylvestre.ledru, marco-c

Differential Revision: https://reviews.llvm.org/D54416

llvm-svn: 346644