Tezt: give help on missing [Uses.t]

What

Adds an optional help string to each Uses.t.

Why

If a test cannot run due to a missing dependency (a Uses.t), lib_tezos can now give a helpful hint to the user indicating how to build the missing dependency.

How

By adding a new optional parameter. Being optional, it forced me to add a dummy () to each Uses.make which is not very elegant.

I also worry that the ~help strings will become out-of-date rendering the proposal less helpful.

Manually testing the MR

Remove evm_kernel.wasm if present on your system, then run a test that uses it:

$ rm -f evm_kernel.wasm
$ tezt --title 'Check L2 blocks consistency of hashes'

and you should obtain a helpful message on how to obtain the required evm_kernel.wasm.

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