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/tests/scripts/generate_bignum_tests.py b/tests/scripts/generate_bignum_tests.py
index eee2f65..9e5db3a 100755
--- a/tests/scripts/generate_bignum_tests.py
+++ b/tests/scripts/generate_bignum_tests.py
@@ -68,13 +68,13 @@
 # the framework
 from mbedtls_dev import bignum_core, bignum_mod_raw # pylint: disable=unused-import
 
-class BignumTarget(test_data_generation.BaseTarget, metaclass=ABCMeta):
-    #pylint: disable=abstract-method
+class BignumTarget(test_data_generation.BaseTarget):
     """Target for bignum (legacy) test case generation."""
     target_basename = 'test_suite_bignum.generated'
 
 
-class BignumOperation(bignum_common.OperationCommon, BignumTarget, metaclass=ABCMeta):
+class BignumOperation(bignum_common.OperationCommon, BignumTarget,
+                      metaclass=ABCMeta):
     #pylint: disable=abstract-method
     """Common features for bignum operations in legacy tests."""
     input_values = [