Add missing check to multipart decrypt

Ensure that the test actually does something, rather than skipping both
parts, also add comment to this effect.

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/tests/suites/test_suite_psa_crypto.function b/tests/suites/test_suite_psa_crypto.function
index a881087..19b687e 100644
--- a/tests/suites/test_suite_psa_crypto.function
+++ b/tests/suites/test_suite_psa_crypto.function
@@ -3494,6 +3494,8 @@
     size_t ad_part_len = 0;
     size_t data_part_len = 0;
 
+    /* Ensure that either one part of the test or the other is done, i.e this
+     * test does something. */
     TEST_ASSERT( do_test_ad_chunked || do_test_data_chunked );
 
     /* Temporary whilst we have algorithms that cannot support chunking */
@@ -3588,6 +3590,10 @@
     size_t ad_part_len = 0;
     size_t data_part_len = 0;
 
+    /* Ensure that either one part of the test or the other is done, i.e this
+     * test does something. */
+    TEST_ASSERT( do_test_ad_chunked || do_test_data_chunked );
+
     /* Temporary whilst we have algorithms that cannot support chunking */
     if( do_test_ad_chunked == 1 )
     {