`modular`: Increase cases when perf is better.

[github-actions]

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_inv() {
        const_for!(BITS in NON_ZERO {
            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 { 6 } else { 20 });
            proptest!(config, |(a: U, m: U)| {
                if let Some(inv) = a.inv_mod(m) {
                    assert_eq!(a.mul_mod(inv, m), U::from(1));

From src/modular.rs:261