Create tests for the parallel-executor

[AurelienFT]

This new module could benefit from TDD (test driven development) and so we should create the tests first. The skeleton of this module already exists but it needs to have a bit more data inside in order to allow better tests scenarios.

Design of parallel-executor : https://www.notion.so/fuellabs/Parallel-block-execution-1502f2293f3180d7b861cc02c510e558?pvs=25#1d02f2293f3180ff8285f0bcb87e26c2

Items that need to be tested :

• ParallelExecutor asks correctly the transactions to TxPool.
  • Given the state of workers thread some contract id must be filtered, the number of gas remaining must be lower, ...
  • Given the answers from transaction pool about the remaining tx or not, the ParallelExecutor must be able to adapt his next requests to the TxPool
• Ordering of transactions must produce a valid block and follow the priority ordering from the pool
• Transactions that require state from other workers must influence the total consumption and taken in the gas left. (time left)
• Coins mustn't create dependencies but a check at the end must ensure that the dependency tree in the block is correct and all coins exists when created
• The worstest case we can find with contract dependency mustn't break our limits rule.