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>
diff --git a/tests/scripts/generate_bignum_tests.py b/tests/scripts/generate_bignum_tests.py
index 091630d..5cb7997 100755
--- a/tests/scripts/generate_bignum_tests.py
+++ b/tests/scripts/generate_bignum_tests.py
@@ -85,8 +85,8 @@
 
 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):