Refactor `test_utils` module in `mithril-common` crate

[jpraynaud]

Why

We want the test_utils module of the mithril-common crate to follow the standard structure that we have applied to the internal crates. This will allow us to remove the test_tools feature in the mithril-common crate.

What

Adapt the test_utils module to follow the new structure for test modules.

How

• Rename the module to test
• Create a double module for test doubles
• Migrate all the dummy functions in the fake_data sub-module:
  • From entities
  • From messages
• Restructure the module which has a lot of sub-modules
• Create an Dev ADR