Summary of changes

This updates the netsocket tests to use our new mbed-ce.dev echo server instead of the mbedcloudtesting.com server (which no longer exists :/ ). This allows these tests to work properly again when run.

Impact of changes

Migration actions required

Documentation

I updated the netsocket test README a bit to clarify some of the docs on creating an echo server. It was already mostly pretty good though.

Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

Test results

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[X] Tests / results supplied as part of this PR

Test run in progress...