Switch to a default value of -1u

Since we explicitly document the value 0xFFFFFFFF or -1u as representing
'result not available', we can use it as a sensible default value
without creating an API change. Use this value instead of introducing a
new verification result value.

Signed-off-by: David Horstmann <david.horstmann@arm.com>

tests/suites/test_suite_ssl.function

@@ -6065,7 +6065,7 @@ void verify_result_without_handshake(void)
 
     uint32_t verify_result = mbedtls_ssl_get_verify_result(&ssl);
 
-    TEST_EQUAL(verify_result, MBEDTLS_X509_VERIFY_NOT_STARTED);
+    TEST_EQUAL(verify_result, 0xFFFFFFFF);
 
 exit:
     mbedtls_ssl_config_free(&conf);