Tezt: move Tag to tezos-test-helpers

Context

The Tag module in tezt-tezos needs to be accessible from other tests, such as P2P tests (e.g. !9188 (merged)). This MR moves the Tag module to tezos-test-helpers which sounds like a reasonable place where to put it.

Note: I remove the list of modules from src/lib_test/dune because it just listed all modules and I couldn't find a good reason to explicitly list modules, and it was confusing to me that just moving tag.ml* did not work (I did not realize the list of modules was explicit).

Manually testing the MR

dune build @check

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).
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