[Test] Difficulty of accessing internal node components

[greged93]

When setting up tests, it can be complicated to access internal components and call methods on them. The current implementation uses a command pattern send to the rollup node, and adding commands can be time consuming (e.g. NetworkHandle command). Verify if it would be possible to evolve the current test pattern in order to get a direct access to the various components of the node.

[frisitano]

Test framework improvement have been implemented in #407