Write a real test for encryption sample code

[tmatsuo]

The current test just invokes the function, but it's very nice if the test actually check if one of the operations described in the doc.

For example, you can invoke:

$ gcloud beta compute disks create example-disk --csek-key-file example-file

with the file created by the current code in the example. When I run the command above with an arbitrary file, it threw the following error:

InvalidKeyFileException: No JSON object could be decoded

IMHO, ensuring it successfully creates a disk by such a test will be a big win.