The old code caused warnings to fire under MSVC, and Clang <3.8.
I could not find a GCC version where it worked, but I assume that it
did at some point.
This new code causes all of MSVC, GCC, Clang, in current versions,
to emit signed/unsigned comparison warning in test like this:
```cpp
TEST_CASE() {
int32_t i = -1;
uint32_t j = 1;
REQUIRE(i != j);
}
```
Where previously only MSVC would emit the warning.
Fixes#1880
60cfaa38fb