Rename test_suite_bignum for consistency with bignum.{h,c}

Align the name of the bignum test suite with the source module (which was
renamed from mpi.c to bignum.c in the PolarSSL 1.x days). This also brings
it into line with the test suites for the low-level bignum interfaces.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

tests/scripts/generate_bignum_tests.py

@@ -85,8 +85,8 @@ def combination_pairs(values: List[T]) -> List[Tuple[T, T]]:
 
 class BignumTarget(test_data_generation.BaseTarget, metaclass=ABCMeta):
     #pylint: disable=abstract-method
-    """Target for bignum (mpi) test case generation."""
-    target_basename = 'test_suite_mpi.generated'
+    """Target for bignum (legacy) test case generation."""
+    target_basename = 'test_suite_bignum.generated'
 
 
 class BignumOperation(BignumTarget, metaclass=ABCMeta):