Prepare user-friendly unit tests for different parts of the code and document them

[xgreenx]

We need to prepare user-friendly unit tests for different parts of the code and document them. The main idea is to provide an easy way for people to create a proof of concept if they find a potential bug.

Since bugs can occur in the GraphQL API, P2P, executor, database, and TxPool, it would be nice if we provided documentation and examples of how to test different scenarios for each area. Examples:

• Set up a small p2p cluster and add some interactions between nodes.
• Upgrade of the network.
• Deploying of the contract and execution of it.
• Running of the simple script.
• Send GraphQL queries from the code.
• Test with a relayer that sends custom data.
• TODO