write spec we got on disk so it can be compared with the expected

While working on https://github.com/bnagy/gapstone/pull/7 and running "go test .", it was not easy to read the test output because of the fmt.Println(fs) statements.
Also, I felt it was easier to have the the spec we got written to disk in case of failure, so it can be diffed and moved to replace the expected.