tests(s2n-quic-core): enable kani for stream try_fit function

[camshaft]

Description of changes:

This change enables a kani proof for the stream frame try_fit function.

Testing

Enabling this proof actually found a bug in the test harness that was overly strict on the fit function. Since this code is in a pretty hot path, we could return a more optimized result at the expense of brute-force computation of each possible length, but it would only save at most a byte or 2 (since our lengths are under 2^16 in practice). As such, I've updated the harness to include the logic to consider intentional unoptimal results.

What's super interesting is libFuzzer didn't catch this, even after running it for quite some time. On the other hand, kani returns a counter example quite quickly:

#[test]
fn try_fit_test_concrete() {
    model(VarInt::new(48).unwrap(), VarInt::new(128).unwrap(), VarInt::new(64).unwrap(), 69);
}

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