Scheduler tests

[maVovk]

Implement unit-tests for KQP scheduler

Necessary scenarios:

1. Fair share calculation

Basic:

• One database, one pool, one query. Demand is under a limit
• Same tree structure but now demand is over a limit and FairShare should be set depending on flag
• Check correct weighted distribution between childs
• More complex structure still passes previous checks

Advanced:

• Structure and limits are changed between UpdateFairShare calls. Queries are added and deleted

Corner-cases:

• Empty root, pool, database etc.
• Zero demand, weight, limit
• Nodes insertion/deletion