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 998709b7, so let's
just hack around it here for the branch.