clang-rename: try to make ClassTestReplacements more reliable

As it failed on e.g.
<http://lab.llvm.org:8011/builders/clang-ppc64be-linux/builds/7089/steps/ninja%20check%201/logs/FAIL%3A%20Clang%20Tools%3A%3AClassTestReplacements.cpp> with:

Trouble iterating over directory '/home/buildbots/ppc64be-clang-test/clang-ppc64be/stage1/tools/clang/tools/extra/test/clang-rename/Output': No such file or directory

A reliable way to trigger the problem locally is to run all clang-rename
tests in parallel in a loop:

for i in $(seq 1 100); do ~/git/llvm/workdir/bin/llvm-lit -v -j15 . || break; done

Change the test script to be more similar to test/Tooling/clang-check.cpp, that
way the above command doesn't fail for me anymore.

llvm-svn: 273941