If an Observe is de-registered, but an unsolicited Observe response is received
before the de-register response is received, then the unsolicited response
failed to get decrypted.
This then makes sure that all things libcoap are properly initialized
before use, and coders realize they need to do this.
Note: coap_new_context() continues to make sure that coap_startup()
is invoked if not done previously, but this may be too late.
See https://core-wg.github.io/oscore/test-spec5.html
Add in new test server oscore-interop-server to provide the necessary
functionality.
Add in script and configuration files to run the tests locally.
Jon Shallow