Fix warnings about signed/unsigned comparisons inside wxMax() and friends.

Fix warnings about signed/unsigned comparisons inside wxMax() and friends.

wxMax, wxMin and wxClip work correctly when called with a mix of signed and
unsigned arguments but give warnings about comparing them when compiled with
g++.

Cast both arguments to the result type, which is defined consistently with
standard C rules for implicit promotion, before comparing them to avoid this.

Also add more tests to check that using these functions in this case doesn't
provoke warnings.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65933 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775