Support variadic functions as parameters

[nathaniel-bennett]

Currently, ctest2 returns an assertion failure when run against a function that contains a variadic function as a parameter. The assertion failure can be found here:

ctest2/src/lib.rs

Line 2107 in 80f9cc4

assert!(!variadic);

This became an issue while introducing NetBSD CI (rust-lang/libc#3965), as NetBSD contains the following function:

pub fn esetfunc(
    cb: ::Option<unsafe extern "C" fn(::c_int, *const ::c_char, ...)>,
) -> ::Option<unsafe extern "C" fn(::c_int, *const ::c_char, ...)>;

For esetfunc to be properly tested, variadic parameters/return values need support.