cpp11-migrate: Use test fixture in preparation for more tests

Existing tests are not changed but the set-up work is moved into a fixture
to simplify forthcoming new tests.



git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@186575 91177308-0d34-0410-b5e6-96231b3b80d8