Use test_ or check_ prefix on functions that verify a result

[nlsandler]

Taking @stephenrkell's feedback in #5 (review)

E.g. rename

int add_large(void) {
    // x = 5l
    x = x + 4294967290l; // this constant is 2^32 - 6
    return (x == 4294967295l);
}

to test_add_large so it's clear the function doesn't just add numbers, it also verifies the result.

In general, use test_ prefix for standalone tests, check_ to validate input from elsewhere