at_c: functional tests for atclient->async_read=true

[JeremyTubongbanua]

Write functional tests to ensure that CRUD operations (put, get, delete) and event operations (notify, monitor) work "correctly" atclient->async_read=true.

With async_read set to true, we allow delete and noop to be ran, but, we return an error if the caller tries to run put, get, or notify because these functions synchronously may need to get encryption keys.

---

Tests to be written:

• Test that putting returns an error when async_read is true
• Test that getting returns an error when async_read is true
• Test that deleting works as normal when async_read is true
• Test that notify returns an error when a async_read is true
• Test that monitor works as normal when async_read is true, when a non-async client sends this monitor client a notification (this test requires 2 clients).