Enhance General Mobile Security Testing chapter

[cpholguera]

We should enhance the chapter with clear guidelines on setting up the scope and expectations with the client. This way we can refer back to a section here whenever a test might get too complex and time consuming.

https://github.com/OWASP/owasp-mstg/blob/master/Document/0x04b-Mobile-App-Security-Testing.md#coordinating-with-the-client

This idea came after discussing #1988