Fix mysterious failure of SupportTests FileCheckTest.Binop
    
    The test would fail in no-asserts release builds using MSVC
    for 64-bit Windows:
    
    Unexpected error message:
    TestBuffer:1:1: error: implicit format conflict between 'FOO' (%u) and '18\0' (%x), need an explicit format specifier
    
    Error message(s) not found:
    {implicit format conflict between 'FOO' (%u) and 'BAZ' (%x), need an explicit format specifier}
    
    It seems a string from a previous test case is finding its way
    into the latter one.
    
    This doesn't reproduce on master anymore after 998709b7d, so let's
    just hack around it here for the branch.