n-api: re-write test_make_callback

This re-writes the test in C by dropping std::vector<napi_value> in
favour of a C99 variable length array, and by dropping the anonymous
namespace in favour of static function declarations.

PR-URL: https://github.com/nodejs/node/pull/19448


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>