C/C++ doctest with separate implementation file and source with tests

factorial.cpp

#define DOCTEST_CONFIG_IMPLEMENTATION_IN_DLL
#include "doctest.h"

//int factorial(int number) { return number <= 1 ? number : factorial(number - 1) * number; }
int factorial(int number) { return number <= 1 ? 1 : factorial(number - 1) * number; }

TEST_CASE("testing the factorial function") {
    CHECK(factorial(0) == 1);
    CHECK(factorial(1) == 1);
    CHECK(factorial(2) == 2);
    CHECK(factorial(3) == 6);
    CHECK(factorial(10) == 3628800);
}

tests.cpp

#define DOCTEST_CONFIG_IMPLEMENT_WITH_MAIN
#include "doctest.h"