tests/mtk/region: Fix `MtkRectangle` leak
`mtk_rectangle_new()` allocates the object dynamically,
but in the "contains-point" test case the allocated object
was not freed. Fix that by creating the object on the stack.
`mtk_rectangle_new()` allocates the object dynamically,
but in the "contains-point" test case the allocated object
was not freed. Fix that by creating the object on the stack.