fix: narrowing conversion in util::compare tests

[pbrit]

You can see what I had while compiling on linux-aarch64 (Dev Container on MacOS) in the screenshot.

clangd suggests to add static_cast<int>, but I decided against it since char shall not be used with negative values. I'm not 100% certain about that for char and negatives are a bit of a tricky thing in the standard.

How do you feel about that change? @JohanMabille @Hind-M

[Hind-M]

Hmm, I would rather do something like:
CHECK(cmp_equal(signed char{ -1 }, signed char{ -1 }));
and keep the tests.

[Hind-M]

Hmm, I would rather do something like: CHECK(cmp_equal(signed char{ -1 }, signed char{ -1 })); and keep the tests.

@pbrit What do you think?