Fix exercise_signature_key for ECDSA

mbedtls_ecdsa_verify fails when the input is all-bits-zero (mbedtls
issue #1792). Use a different input.
diff --git a/tests/suites/test_suite_psa_crypto.function b/tests/suites/test_suite_psa_crypto.function
index 446af5a..cb6da7f 100644
--- a/tests/suites/test_suite_psa_crypto.function
+++ b/tests/suites/test_suite_psa_crypto.function
@@ -189,7 +189,7 @@
                                    psa_key_usage_t usage,
                                    psa_algorithm_t alg )
 {
-    unsigned char payload[16] = {0};
+    unsigned char payload[16] = {1};
     size_t payload_length = sizeof( payload );
     unsigned char signature[256] = {0};
     size_t signature_length = sizeof( signature );