Fix harmless signed/unsigned comparison warning in a test.

Don't compare int with unsigned to avoid warnings that were introduced by the
changes of r75940.

See #15980.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76058 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775