Split test generator base class

The class BaseTarget served two purposes:
- track test cases and target files for generation
- provide an abstract base class for individual test groups

Splitting these allows decoupling these two and to have further common
superclasses across targets.

No intended change in generated test cases.

Signed-off-by: Janos Follath <janos.follath@arm.com>
diff --git a/scripts/mbedtls_dev/test_data_generation.py b/scripts/mbedtls_dev/test_data_generation.py
index eec0f9d..3d703ee 100644
--- a/scripts/mbedtls_dev/test_data_generation.py
+++ b/scripts/mbedtls_dev/test_data_generation.py
@@ -25,6 +25,7 @@
 import os
 import posixpath
 import re
+import inspect
 
 from abc import ABCMeta, abstractmethod
 from typing import Callable, Dict, Iterable, Iterator, List, Type, TypeVar
@@ -35,12 +36,8 @@
 T = TypeVar('T') #pylint: disable=invalid-name
 
 
-class BaseTarget(metaclass=ABCMeta):
-    """Base target for test case generation.
-
-    Child classes of this class represent an output file, and can be referred
-    to as file targets. These indicate where test cases will be written to for
-    all subclasses of the file target, which is set by `target_basename`.
+class BaseTest(metaclass=ABCMeta):
+    """Base class for test case generation.
 
     Attributes:
         count: Counter for test cases from this class.
@@ -48,8 +45,6 @@
             automatically generated using the class, or manually set.
         dependencies: A list of dependencies required for the test case.
         show_test_count: Toggle for inclusion of `count` in the test description.
-        target_basename: Basename of file to write generated tests to. This
-            should be specified in a child class of BaseTarget.
         test_function: Test function which the class generates cases for.
         test_name: A common name or description of the test function. This can
             be `test_function`, a clearer equivalent, or a short summary of the
@@ -59,7 +54,6 @@
     case_description = ""
     dependencies = [] # type: List[str]
     show_test_count = True
-    target_basename = ""
     test_function = ""
     test_name = ""
 
@@ -121,6 +115,20 @@
         """
         raise NotImplementedError
 
+
+class BaseTarget:
+    """Base target for test case generation.
+
+    Child classes of this class represent an output file, and can be referred
+    to as file targets. These indicate where test cases will be written to for
+    all subclasses of the file target, which is set by `target_basename`.
+
+    Attributes:
+        target_basename: Basename of file to write generated tests to. This
+            should be specified in a child class of BaseTarget.
+    """
+    target_basename = ""
+
     @classmethod
     def generate_tests(cls) -> Iterator[test_case.TestCase]:
         """Generate test cases for the class and its subclasses.
@@ -132,7 +140,7 @@
         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:
+        if issubclass(cls, BaseTest) and not inspect.isabstract(cls):
             yield from cls.generate_function_tests()
         for subclass in sorted(cls.__subclasses__(), key=lambda c: c.__name__):
             yield from subclass.generate_tests()