Add tests for spurious certs in the chain

We have code to skip them but didn't have explicit tests ensuring they are
(the corresponding branch was never taken).

While at it, remove extra copy of the chain in server10*.crt, which was
duplicated for no reason.

tests/data_files/server7_spurious_int-ca.crt

@@ -0,0 +1,65 @@
+-----BEGIN CERTIFICATE-----
+MIIDwjCCAaqgAwIBAgIBEDANBgkqhkiG9w0BAQsFADBIMQswCQYDVQQGEwJOTDER
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+-----END CERTIFICATE-----
+-----BEGIN CERTIFICATE-----
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+-----END CERTIFICATE-----
+-----BEGIN CERTIFICATE-----
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+-----END CERTIFICATE-----