`gcd`: Increase cases when perf is better.

[github-actions[bot]]

On 2022-06-11 @recmo wrote in ac23d9d “Merge pull request #140 from recmo/test-time”:

Increase cases when perf is better.

    #[test]
    fn test_gcd_identities() {
        const_for!(BITS in SIZES {
            const LIMBS: usize = nlimbs(BITS);
            type U = Uint<BITS, LIMBS>;
            // TODO: Increase cases when perf is better.
            let mut config = Config::default();
            config.cases = min(config.cases, if BITS > 500 { 3 } else { 10 });
            proptest!(config, |(a: U, b: U)| {
                let g = a.gcd(b);
                assert_eq!(b.gcd(a), g);

From src/gcd.rs:61