WIP: Move Michelson ADD opcode test from Pytest to Alcotest

Context

This was a little project given to me by @galfour for evaluation purposes.

Manually testing the MR

Run make test-unit to run all Alcotest tests - all tests should pass. Then break tests_python/contracts/opcodes/add.tz (perhaps by changing one of the PUSH int 2;s to PUSH int 3;), and re-run make test-unit - the "add" test should fail.

Extra notes

It should be straightforward to move many of the tests from tests_python/tests/test_contract_opcodes.py to Alcotest without many changes. I would have just moved them all, but I couldn't figure out how to read Michelson's output and wanted to sign off for today. I'd be happy to finish that up tomorrow!