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/bignum_mod.py b/scripts/mbedtls_dev/bignum_mod.py
index 2bd7fbb..a604cc0 100644
--- a/scripts/mbedtls_dev/bignum_mod.py
+++ b/scripts/mbedtls_dev/bignum_mod.py
@@ -14,12 +14,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from abc import ABCMeta
-
 from . import test_data_generation
 
-class BignumModTarget(test_data_generation.BaseTarget, metaclass=ABCMeta):
-    #pylint: disable=abstract-method
+class BignumModTarget(test_data_generation.BaseTarget):
+    #pylint: disable=abstract-method, too-few-public-methods
     """Target for bignum mod test case generation."""
     target_basename = 'test_suite_bignum_mod.generated'