secret-store fix warning in tests

[niklasad1]

Fixes:

warning: unused return value of `std::iter::Iterator::collect` which must be used
   --> secret_store/src/key_server_cluster/math.rs:590:3
    |
590 |           (0..n).map(|i| (0..n).map(|j| if i != j {
    |  _________^
591 | |             assert!(keys_verification(t, &derived_point, &id_numbers[i], &secrets1[j][i], &secrets2[j][i], &publics[j]).unwrap());
592 | |         }).collect::<Vec<_>>()).collect::<Vec<_>>();
    | |____________________________________________________^
    |
    = note: #[warn(unused_must_use)] on by default
    = note: if you really need to exhaust the iterator, consider `.for_each(drop)` instead

[twittner]

Personally I would find nested for loops easier to read, i.e.

for i in 0 .. n {
	for j in 0 .. n {
		if i != j {
			...
		}
	}
}