Tests: Add Metaclass for BitcoinTestFramework

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

test/functional/test_framework/test_framework.py

@@ -41,7 +41,28 @@ TEST_EXIT_PASSED = 0
 TEST_EXIT_FAILED = 1
 TEST_EXIT_SKIPPED = 77
 
-class BitcoinTestFramework():
+
+class BitcoinTestMetaClass(type):
+    """Metaclass for BitcoinTestFramework.
+
+    Ensures that any attempt to register a subclass of `BitcoinTestFramework`
+    adheres to a standard whereby the subclass overrides `set_test_params` and
+    `run_test` but DOES NOT override either `__init__` or `main`. If any of
+    those standards are violated, a ``TypeError`` is raised."""
+
+    def __new__(cls, clsname, bases, dct):
+        if not clsname == 'BitcoinTestFramework':
+            if not ('run_test' in dct and 'set_test_params' in dct):
+                raise TypeError("BitcoinTestFramework subclasses must override "
+                                "'run_test' and 'set_test_params'")
+            if '__init__' in dct or 'main' in dct:
+                raise TypeError("BitcoinTestFramework subclasses may not override "
+                                "'__init__' or 'main'")
+
+        return super().__new__(cls, clsname, bases, dct)
+
+
+class BitcoinTestFramework(metaclass=BitcoinTestMetaClass):
     """Base class for a bitcoin test script.
 
     Individual bitcoin test scripts should subclass this class and override the set_test_params() and run_test() methods.
@@ -432,6 +453,7 @@ class BitcoinTestFramework():
         for i in range(self.num_nodes):
             initialize_datadir(self.options.tmpdir, i)
 
+
 class SkipTest(Exception):
     """This exception is raised to skip a test"""
     def __init__(self, message):