Merge #12856: Tests: Add Metaclass for BitcoinTestFramework

Summary:
c9cce0a Tests: Add Metaclass for BitcoinTestFramework (Will Ayd)

Pull request description:

BitcoinTestFramework instructs developers in its docstring to override
`set_test_params` and `run_test` in subclasses while being sure NOT to
override `__init__` and `main` . This change adds a metaclass to ensure
that developers adhere to that protocol, raising a ``TypeError`` in
instances where they have not.

closes #12835

Tree-SHA512: 5a47a7ead1f18361138cad4374747c4a8f29d25506f7b2c2a8c1c966a0b65e5ccf7317f9a078df8680fdab5d3fb71fee46a159c9f381878a3683c1e9f874abbe

Backport of Core PR12856

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, nakihito

Reviewed By: nakihito

Differential Revision: https://reviews.bitcoinabc.org/D6353