Etherlink/Tezt: End to end tezt test to test the multichain feature

What

This MR introduces tools to write a multichain test in the etherlink tezt library.

Write a simple test to verify that it works (produce a block and verify that the block number was incremented)

Why

This MR is a setup for future MR that'll need an interface to test the real multichain feature. As for the moment the multichain feature is not implemented yet in the kernel but when it'll come we can use the functions in this MR and also complexify the current test.

How

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR