[CN-Test-Gen] Flag to toggle using builtin malloc versus cn_fl_malloc

[ZippeyKeys12]

With the implementation of a general allocator (#863), we can disconnect the testing's dependency on malloc. However, this introduces a slowdown, since our allocator is not as fast (though still quite good). If we do want to support testing without malloc, we'd want a flag to allow the user to choose between the two.

I'm not sure when (if ever) you'd want to run the full testing setup without malloc though.