Add Socket test plan

I'm submitting a test plan for Mbed OS sockets, so that we have
reference that describes wanted behaviour, required test environment,
and priorities of test cases.

This servers as a both documentation and specification. It needs to live
with source code, as opposed to Handbook, because it will be extend, modified
and fixed for faster cycle than what should be done for Handbook.

Part of this document (How to run -section) will be submitted to Handbook
with links to this.

Author: Seppo Takalo