tests: ssl: allow more groups in conf_group()

Previously 3 different groups were allowed, but since the removal of
secp192r1 and secp224r1 only secp256r1 was left. This commit adds
other 2 options.

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>

tests/suites/test_suite_ssl.function

@@ -3538,6 +3538,8 @@ exit:
 void conf_group()
 {
     uint16_t iana_tls_group_list[] = { MBEDTLS_SSL_IANA_TLS_GROUP_SECP256R1,
+                                       MBEDTLS_SSL_IANA_TLS_GROUP_SECP384R1,
+                                       MBEDTLS_SSL_IANA_TLS_GROUP_SECP521R1,
                                        MBEDTLS_SSL_IANA_TLS_GROUP_NONE };
 
     mbedtls_ssl_config conf;