Add some more x509_verify tests

- trust chain of depth 0
- invalid signature
- trust chain of depth 2
- multiple trusted CA's

tests/data_files/server8.key

@@ -0,0 +1,8 @@
+-----BEGIN EC PARAMETERS-----
+BggqhkjOPQMBAQ==
+-----END EC PARAMETERS-----
+-----BEGIN EC PRIVATE KEY-----
+MF8CAQEEGItTogpE7AOnjvYuTqm+9OabmsX02XKIAqAKBggqhkjOPQMBAaE0AzIA
+BH0AoQyUhPABS38y67uEVs4O3RXmKKrBdUR7/L2QPB8EC2p5fQcsej6EFasvlTdJ
+/w==
+-----END EC PRIVATE KEY-----