Tezt: extract helpers from FA1.2 tests

Context

I want to use the mini_scenarios/fa12_reference contract in a future merge request for tests. The client_fa12.ml implements multiple helpers I'd like to use. The helpers will be used in tezt/tests/evm_rollup.ml so there will be a dependency between the two test files, please let met know if it's a no-go. Introduced an helper file.

Manually testing the MR

CI should be enough.

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