improve e2e test experience

[AlexD10S]

• Make command less verbose: pop test contract --e2e instead of pop test contract --features e2e-tests

• In PR refactor: run substrate-contracts-node in pop up contract if it does not exist #206, the functionality to download and run the substract-contracts-node for the pop up contract was introduced, which runs if no development chain is specified by the user for contract deployment. We should refactor this logic to reuse parts of it in the pop test contract --e2e.

  Specifically, we should:
  1. Download the binary.
  2. Automatically set the variable CONTRACTS_NODE_PATH to the path.
  This approach provides an easy alternative to the solution proposed in chore: set CONTRACTS_NODE_PATH env variable for e2e tests #209.