Split generate_tests to reduce code complexity

Previous implementation mixed the test case generation and the
recursive generation calls together. A separate method is added to
generate test cases for the current class' test function. This reduces
the need to override generate_tests().

Signed-off-by: Werner Lewis <werner.lewis@arm.com>

scripts/mbedtls_dev/test_generation.py

@@ -25,7 +25,7 @@ import posixpath
 import re
 
 from abc import ABCMeta, abstractmethod
-from typing import Callable, Dict, Iterable, List, Type, TypeVar
+from typing import Callable, Dict, Iterable, Iterator, List, Type, TypeVar
 
 from mbedtls_dev import build_tree
 from mbedtls_dev import test_case
@@ -91,16 +91,31 @@ class BaseTarget(metaclass=ABCMeta):
         return tc
 
     @classmethod
-    def generate_tests(cls):
-        """Generate test cases for the target subclasses.
+    @abstractmethod
+    def generate_function_tests(cls) -> Iterator[test_case.TestCase]:
+        """Generate test cases for the test function.
 
-        During generation, each class will iterate over any subclasses, calling
-        this method in each.
-        In abstract classes, no tests will be generated, as there is no
-        function to generate tests for.
-        In classes which do implement a test function, this should be overridden
-        and a means to use `create_test_case()` should be added.
+        This will be called in classes where `test_function` is set.
+        Implementations should yield TestCase objects, by creating instances
+        of the class with appropriate input data, and then calling
+        `create_test_case()` on each.
         """
+        pass
+
+    @classmethod
+    def generate_tests(cls) -> Iterator[test_case.TestCase]:
+        """Generate test cases for the class and its subclasses.
+
+        In classes with `test_function` set, `generate_function_tests()` is
+        used to generate test cases first.
+        In all classes, this method will iterate over its subclasses, and
+        yield from `generate_tests()` in each.
+
+        Calling this method on a class X will yield test cases from all classes
+        derived from X.
+        """
+        if cls.test_function:
+            yield from cls.generate_function_tests()
         for subclass in sorted(cls.__subclasses__(), key=lambda c: c.__name__):
             yield from subclass.generate_tests()