C, how can a macro register a unittest for execution?

How can libraries such as GTest use a single macro G_TEST(...) to both define the function and register it as a test function? I'm looking for a way to do the same for my C99 project.

Example

TEST_MACRO(mytest)
{
    printf("Hello, world\n");
    return 0;
}
TEST_MACRO(mytest2)
{
    printf("Hello, world 2\n");
    return 2;
}

void main(int argc, char** argv)
{
    RUN_TESTS();
}

Can't figure out how to register the tests so they are known from RUN_TESTS()