ci: check for s2n_array_len in loop bounds

[lrstewart]

Description of changes:

Whenever I'm writing a test of the form:

struct {
    /* some variable arguments */
} test_cases[] = { /* some test cases */ };
for (size_t i = 0; i < s2n_array_len(test_cases); i++) {

I'm always worried that I'm going to do "sizeof" instead of "s2n_array_len". Best case, that would cause the test to iterate over test cases that don't exist, probably failing, at a minimum probably accessing invalid memory. But worst case, the test silently doesn't execute some test cases. I've made this mistake before.

So I added a grep_simple_mistakes that only allows sizeof in loops for variables with names indicating that's probably safe. I'd love to try to expand it to all uses of sizeof, but that's a MUCH larger set of mistakes. Loops are also what I'm most concerned about: it's a common pattern, and it's very hard to detect the mistake. Other sizeofs are generally used in assertions.

Testing:

Tests continue to pass. Here's an example of a violation:

Warning: sizeof is only valid for arrays of chars or uint8_ts. Use s2n_array_len for other types, or append "bytes", "data", or "u8" to your variable name for clarity. Warning in /home/lrstewart/Code/s2n-tls/tests/unit/s2n_resume_test.c:
408:        for (size_t i = 0; i < sizeof(ems_state); i++) {

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[lrstewart]

Luckily this and line 720 are the only existing mistakes I found. From my testing, they currently pass without triggering an invalid memory access because they never iterate all the way through the loop. Currently, both always bail on the first kem group.