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.crt b/tests/data_files/server7.crt
new file mode 100644
index 0000000..5040bec
--- /dev/null
+++ b/tests/data_files/server7.crt
@@ -0,0 +1,14 @@
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+-----END CERTIFICATE-----