Add some more x509_verify tests

- trust chain of depth 0
- invalid signature
- trust chain of depth 2
- multiple trusted CA's
diff --git a/tests/data_files/server7.key b/tests/data_files/server7.key
new file mode 100644
index 0000000..844bb44
--- /dev/null
+++ b/tests/data_files/server7.key
@@ -0,0 +1,5 @@
+-----BEGIN EC PRIVATE KEY-----
+MF8CAQEEGO82j8OXBoUhVyauCA8XZ288l595u7BXWqAKBggqhkjOPQMBAaE0AzIA
+BMtC4d2X4RMAYgvI5iXxIPDRsQg6hxLc/oT4GLN+4Q8/cEzqGPgiQ7RFHA3nYBQq
+ag==
+-----END EC PRIVATE KEY-----