enforce usage of the get_tests comptool interface as ComparisonTestFramework method

In practice all-and-only subclasses of ComparisonTestFramework implement get_tests.
This commit enforces this relation.

qa/rpc-tests/test_framework/comptool.py

@@ -30,6 +30,8 @@ using a script provided.
 To use, create a class that implements get_tests(), and pass it in
 as the test generator to TestManager.  get_tests() should be a python
 generator that returns TestInstance objects.  See below for definition.
+
+In practice get_tests is always implemented on a subclass of ComparisonTestFramework.
 '''
 
 # TestNode behaves as follows:

qa/rpc-tests/test_framework/test_framework.py

@@ -203,3 +203,6 @@ class ComparisonTestFramework(BitcoinTestFramework):
             extra_args=[['-debug', '-whitelist=127.0.0.1']] * self.num_nodes,
             binary=[self.options.testbinary] +
             [self.options.refbinary]*(self.num_nodes-1))
+
+    def get_tests(self):
+        raise NotImplementedError