Bulk assigner tests update (#2277)

Addressed comments from
https://github.com/paritytech/polkadot-sdk/pull/2262

Also added tests to cover the new cases:
- Test that assignments are served correctly. E.g. two equal assignments
will be served as ABABAB
- Have a test that checks that core is shared fairly, even in case of
`ratio` not being divisible by `step`