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_common.py b/scripts/mbedtls_dev/bignum_common.py
index 8b11bc2..ba30be4 100644
--- a/scripts/mbedtls_dev/bignum_common.py
+++ b/scripts/mbedtls_dev/bignum_common.py
@@ -17,6 +17,8 @@
 from abc import abstractmethod
 from typing import Iterator, List, Tuple, TypeVar
 
+from . import test_data_generation
+
 T = TypeVar('T') #pylint: disable=invalid-name
 
 def invmod(a: int, n: int) -> int:
@@ -63,8 +65,7 @@
     """Return all pair combinations from input values."""
     return [(x, y) for x in values for y in values]
 
-
-class OperationCommon:
+class OperationCommon(test_data_generation.BaseTest):
     """Common features for bignum binary operations.
 
     This adds functionality common in binary operation tests.