[clang/test] Fix a flaky unittest on windows

Summary:
The append operator on a shell command for quick command-line is
causing trouble on windows. [NFC]
The easiest way to fix them is to avoid using them.

This patch is an attempt to fix this broken build bot:
  clang-x86-win2008-selfhost

http://lab.llvm.org:8011/builders/clang-x86-win2008-selfhost/builds/9523

Reviewers: rnk

Subscribers: cfe-commits, chrisha

Differential Revision: https://reviews.llvm.org/D23105

llvm-svn: 277576