Add module for bignum_core test generation

Separate file is added for classes used to generate cases for tests
in bignum_core.function. Common elements of the BignumOperation class
are added to classes in a new common file, for use across files.

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

scripts/mbedtls_dev/test_data_generation.py

@@ -148,6 +148,7 @@ class TestGenerator:
         self.targets.update({
             subclass.target_basename: subclass.generate_tests
             for subclass in BaseTarget.__subclasses__()
+            if subclass.target_basename
         })
 
     def filename_for(self, basename: str) -> str: