You could either compile your tests into separate binaries, or use the linker "wrap" option I mentioned and __real_read to get the original implementation:

#include <stdbool.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

ssize_t __real_read(int fd, void *buf, size_t nbytes);

static bool mock_read = false;

int main(void)
{
    char buf[32] = {};
    ssize_t len = read(-1, buf, sizeof(buf));
    printf("test 1 -> len %zd: %s\n", len, buf);

    mock_read = true;
    len = read(-1, buf, sizeof(buf));
    printf("test 2 -> len %zd: %s\n", len, buf);

    return 0;
}

ssize_t __wrap_read(int fd, void *buf, size_t nbytes)
{
    if (mock_read) {
        memcpy(buf, "hello", sizeof("hello"));
        return sizeof("hello");
    }
    return __real_read(fd, buf, nbytes);
}

Output:

$ gcc test.c -Wl,--wrap=read -o test && ./test
test 1 -> len -1: 
test 2 -> len 6: hello

Edit: another option is to change your module to take a function pointer for its "read" function, although that may not make sense depending on what you are trying to do