Add sanity check for FFDH key excercise

Signed-off-by: Przemek Stekiel <przemyslaw.stekiel@mobica.com>
diff --git a/tests/src/psa_exercise_key.c b/tests/src/psa_exercise_key.c
index 2656deb..dc7c971 100644
--- a/tests/src/psa_exercise_key.c
+++ b/tests/src/psa_exercise_key.c
@@ -789,6 +789,9 @@
             TEST_EQUAL(1 + 2 * PSA_BITS_TO_BYTES(bits), exported_length);
             TEST_EQUAL(exported[0], 4);
         }
+    } else
+    if (PSA_KEY_TYPE_IS_DH_PUBLIC_KEY(type) || PSA_KEY_TYPE_IS_DH_KEY_PAIR(type)) {
+        TEST_LE_U(exported_length, PSA_BITS_TO_BYTES(bits));
     } else {
         (void) exported;
         TEST_ASSERT(!"Sanity check not implemented for this key type");