[pallet-broker] add tests for renewing leases (#4099)

The first test proves that parachains who were migrated over on a legacy
lease can renew without downtime.

The exception is if their lease expires in period 0 - aka within
`region_length` timeslices after `start_sales` is called. The second
test is designed such that it passes if the issue exists and should be
fixed.
This will require an intervention on Kusama to add these renewals to
storage as it is too tight to schedule a runtime upgrade before the
start_sales call. All leases will still have at least two full regions
of coretime.