Bump TLS-Attacker from 5.3.0 to v6.3.4 #979

Job	Run time
CTinyDTLS_Client_psk	8m 42s
CTinyDTLS_Server_psk	6m 48s
ETinyDTLS_Server_psk	6m 58s
JSSE-12-0-2_Server_ecdhe_cert_req	22m 29s
ETinyDTLS_Client_psk	5m 49s
JSSE-12-0-2_Client_ecdhe_cert	43m 46s
Mbedtls-2-26-0_Server_all_cert_req	14m 0s
OpenSSL-1-1-1k_Server_all_cert_req	10m 31s
Mbedtls-2-26-0_Client_dhe_ecdhe_rsa_eneg	6m 37s
OpenSSL-1-1-1k_Client_dhe_ecdhe_rsa_eneg	5m 56s
PionDTLS-2-0-9_Client_psk	6m 44s
PionDTLS-2-0-9_Server_psk	10m 21s
Scandium-2-0-0-M16_Server_ecdhe_cert_req	32m 29s
Scandium-2-0-0-M16_Client_ecdhe_cert	10m 14s
WolfSSL-4-7-1r_Client_psk	8m 16s
WolfSSL-5-7-6_Server_psk	9m 39s
WolfSSL-4-7-1r_Server_psk	12m 20s
WolfSSL-5-7-6_Client_psk	8m 59s
	3h 50m 38s

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