glcpp: Fix test suite to avoid flagging failed tests as valgrind errors.

We recently added several tests that intentionally trigger
preprocessor errors. During valgrind-based testing, our test script
was noticing the non-zero return value from the preprocessor and
incorrectly flagging the valgrind-based test as failing.

To fix this, we make valgrind return an error code that is otherwise
unused by the preprocessor.

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