Improve readability of null-argument tests

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/suites/test_suite_ecp.function b/tests/suites/test_suite_ecp.function
index ced4ca3..c8be4e5 100644
--- a/tests/suites/test_suite_ecp.function
+++ b/tests/suites/test_suite_ecp.function
@@ -1401,17 +1401,21 @@
         TEST_EQUAL(export_grp.id,
                    mbedtls_ecp_keypair_get_group_id(&key));
 
-        /* Test null arguments */
+        /* Test null arguments: grp only */
         mbedtls_ecp_group_free(&export_grp);
-        mbedtls_mpi_free(&export_d);
-        mbedtls_ecp_point_free(&export_Q);
         mbedtls_ecp_group_init(&export_grp);
-        mbedtls_mpi_init(&export_d);
-        mbedtls_ecp_point_init(&export_Q);
         TEST_EQUAL(mbedtls_ecp_export(&key, &export_grp, NULL, NULL), 0);
         TEST_EQUAL(mbedtls_ecp_group_cmp(&key.grp, &export_grp), 0);
+
+        /* Test null arguments: d only */
+        mbedtls_mpi_free(&export_d);
+        mbedtls_mpi_init(&export_d);
         TEST_EQUAL(mbedtls_ecp_export(&key, NULL, &export_d, NULL), 0);
         TEST_EQUAL(mbedtls_mpi_cmp_mpi(&key.d, &export_d), 0);
+
+        /* Test null arguments: Q only */
+        mbedtls_ecp_point_free(&export_Q);
+        mbedtls_ecp_point_init(&export_Q);
         TEST_EQUAL(mbedtls_ecp_export(&key, NULL, NULL, &export_Q), 0);
         TEST_EQUAL(mbedtls_ecp_point_cmp(&key.Q, &export_Q), 0);
     }