Computationally prove dispatche queue processing will take place

[techraed]

Some works on removing constant time limits of executing extrinsics and processing dispatches in the queue (i.e., time for on_idle) are planned. We need to make tests that prove an ability to process dispatches in the queue in any case of extrinsics load, when there are no constant time limits

[techraed]

Will be implemented, If only decide to implement #913

[breathx]

That wasn't fully motivated initially, but will be totally unused once #1531 solved