Fix using weak references to incomplete classes.

Fix using weak references to incomplete classes.

wxWeakRef can work with forward-declared classes provided dynamic_cast<> is
available but this wasn't detected as being the case due to the use of the
obsolete HAVE_DYNAMIC_CAST in its code.

Replace HAVE_DYNAMIC_CAST with !wxNO_RTTI to fix this.

Also add a unit test checking that this does work.

Closes #11916.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63986 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775