[Tech]: Liquidity Proxy: rework tests

[wer1st]

Problem statement

Sora network goes ahead have several liquidity sources (almost 4). The new ALT mechanism is developed for liquidity proxy to work with all sources, but unit tests of liquidity proxy are too old and work with limited number of liquidity sources.

Description

It is necessary to rewrite unit tests for liquidity proxy pallet.

Definition of Done

• [ ] Write new unit tests that consider new logic and all liquidity sources.
• [ ] Use Runtime instead of Mock
• [ ] Delete Mock
• [ ] Delete mock_liquidity_source pallet

Requirements

• Tests check at least as much functionality/cases as they were prior to the rework.