gh-125985: Fix `cmodule_function()` scaling benchmark (#128460)

Add a separate benchmark that measures the effect of
`_PyObject_LookupSpecial()` on scaling.

In the process of cleaning up the scaling benchmarks for inclusion, I
unintentionally changed the "cmodule_function" benchmark to pass an
`int` to `math.floor()` instead of a `float`, which causes it to use the
`_PyObject_LookupSpecial()` code path. `_PyObject_LookupSpecial()` has
its own scaling issues that we want to measure separately from calling a
function on a C module.