Make warnings in assertions fire for GCC/Clang again

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

tests/SelfTest/IntrospectiveTests/InternalBenchmark.tests.cpp

@@ -300,7 +300,7 @@ TEST_CASE("analyse", "[approvals][benchmark]") {
     CHECK(analysis.outliers.low_severe == 0);
     CHECK(analysis.outliers.high_mild == 0);
     CHECK(analysis.outliers.high_severe == 0);
-    CHECK(analysis.outliers.samples_seen == samples.size());
+    CHECK(analysis.outliers.samples_seen == static_cast<int>(samples.size()));
 
     CHECK(analysis.outlier_variance < 0.5);
     CHECK(analysis.outlier_variance > 0);