Unit test for a function in a function using CppUTeset

How a unit test can be made for this kind of structure?
How to change the call so function a() will call function mock_b() instead of function b().

// source.h
int b(int x);
bool a();


// source.cpp
int b(int x){return x;}
bool a(){return (b(5) > 0);}


// tester.cpp
#include "source.h"
int mock_b(int x){return -8;}

unit_test(){
    assert(true == a());
    b = mock_b;                 //How this replacement can be done?
    assert(false == a());
}