tests: Validate structure of request body before processing

[roshnet]

Description

As of now, for POST /login and POST /signup, the request body needs to have a specific structure.
If that structure of request body is different from what is assumed in handling it, an exception is caused, which can cause entire server to halt, and other undesired effects.

Fix

This can be prevented by validating request body structure, and returning appropriate response if the structure is corrupt.