force clients to set tls.Config.InsecureSkipVerify when using mint

mint doesn't verify the certificate chain. This change forces users of
quic-go to acknowledge mint's insecure behavior by explicitely setting
InsecureSkipVerify.

integrationtests/self/handshake_test.go

@@ -79,7 +79,8 @@ var _ = Describe("Handshake tests", func() {
 	})
 
 	Context("Certifiate validation", func() {
-		for _, v := range []protocol.VersionNumber{protocol.Version39, protocol.VersionTLS} {
+		// no need to run these tests with TLS. mint doesn't do certificate verification
+		for _, v := range protocol.SupportedVersions {
 			version := v
 
 			Context(fmt.Sprintf("using %s", version), func() {