Tezlink/Node: fix block info lvl 2 mock when non tz1 bootstrap account

What

We fix a pb in the mock introduced to index bootstrap accounts.

Why

It's broken at the moment if there are non tz1 bootstrap accounts

How

We just ignore non tz1 addresses. We're about to start a project to index bootstrap smart contracts anyway, so a proper fix will come.

Manually testing the MR

We added a very simple test that just checks that in the presence of a smart contract the RPC still answers. No real check on the return values, they are mocked anyway.

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