ECDH: improve comment in test function

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>

tests/suites/test_suite_ecdh.function

@@ -422,6 +422,10 @@ void ecdh_exchange_calc_secret(int grp_id,
     shared_secret_length = 0;
 
     /* Try again with an output buffer that's larger. */
+    /* Note: this doubles as a weak test for re-using a context. A proper test
+     * for static ECDH with context re-use should also import multiple peer keys
+     * in a row and compute the corresponding shared secrets, but the below is
+     * already better then nothing. */
     TEST_CALLOC(buf, min_buf_size + 1);
     TEST_EQUAL(0,
                mbedtls_ecdh_calc_secret(