Disable wxUIActionSimulator in wxGTK3 if --without-xtest was given

This class can't work without XTest support with GTK+ 3, so just silently
disable it if XTest was explicitly disabled by user.

configure

@@ -33659,6 +33659,8 @@ $as_echo "yes" >&6; }
 
 
 fi
+        elif test "$WXGTK3" = 1; then
+                                                wxUSE_UIACTIONSIMULATOR=no
         fi
     fi
 

configure.in

@@ -6379,6 +6379,11 @@ if test "$wxUSE_UIACTIONSIMULATOR" = "yes" ; then
                     wxUSE_XTEST="no"
                 ]
             )
+        elif test "$WXGTK3" = 1; then
+            dnl As per above, wxUIActionSimulator can't be used in this case,
+            dnl but there is no need to warn, presumably the user knows what
+            dnl he's doing if --without-xtest was explicitly specified.
+            wxUSE_UIACTIONSIMULATOR=no
         fi
     fi