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I found an improvement that is worth posting as an answer to my own question.

One thing I was not comfortable with was coming up with test cases and figuring out how many assignments a format string should have by just analyzing it visually. So I had to find a second way to do achieve the same thing that I'm trying to do.

I did it by writing a function that takes a format string as argument, creates a file called `warning.c` which only contains a main function with a single scanf call with the format string specified. Then the function executes a compile command and counts the number of warnings, because there is one warning for each argument that is missing, and I'm giving no arguments. Here is the code for that:

```
size_t get_no_warnings(const char *fmt)
{
    FILE *fp = fopen("warning.c", "w");
    if(!fp) {
        perror("Error opening file");
        exit(EXIT_FAILURE);
    }

    fprintf(fp, "#include <stdio.h>\n\nint main(void) {\n\tscanf(\"%s\");\n}\n", fmt);
    fclose(fp);

    fp = popen("gcc warning.c -Wall -Wextra -pedantic 2>&1 | grep \"warning: format\" | wc -l", "r");

    if(!fp) {
        perror("Error executing command");
        exit(EXIT_FAILURE);
    }

    char output[100];
    fgets(output, sizeof(output), fp);
    size_t ret;
    if(sscanf(output, "%zu", &ret) != 1) {
	perror("Error reading output");
	exit(EXIT_FAILURE);
    }
    return ret;
}
```

With that, I can modify my original code like this. I can change

    assert(count_assignments(tc.fmt) == tc.n);

to

    assert(count_assignments(tc.fmt) == get_no_warnings(tc.fmt));

I have not made up my mind yet if I should remove the `n` field from `test_case`. Would look better, but I'm not sure if it's worth the effort. Plus that I assume it's a good thing that I also check if my manual inspection is correct or not. If it's not, there's something about format strings that I don't understand properly.