CA: Use unknown extension in "unsupported extension" test. (#2914)

The test previously used an invalid encoding of the CT poison extension
(the value was empty, but a valid CT poison extension has a NULL value).
In preparation for testing specifically how the CT poison extension is
handled, change the test to use a different extension instead.

ca/ca_test.go

@@ -77,7 +77,12 @@ var (
 	// CSR generated by Go:
 	// * Random public key
 	// * CN = not-example.com
-	// * Includes an extensionRequest attribute for the CT Poison extension (not supported)
+	// * Includes an extensionRequest attribute for an unknown extension with an
+	//   empty value. That extension's OID, 2.25.123456789, is on the UUID arc.
+	//   It isn't a real randomly-generated UUID because Go represents the
+	//   components of the OID as 32-bit integers, which aren't large enough to
+	//   hold a real 128-bit UUID; this doesn't matter as far as what we're
+	//   testing here is concerned.
 	UnsupportedExtensionCSR = mustRead("./testdata/unsupported_extension.der.csr")
 
 	// CSR generated by Go: