Reactivate EMAC Greentea test

[multiplemonomials]

Summary of changes

This PR reactivates the emac test (under connectivity/netsocket/tests/TESTS/network/emac). This test is a bit complex because it works by communicating with a second board running a CTP sever application. To avoid build system cancer, I had to move this application into its own repository. I also moved the common code used by both the CTP server and the Mbed test into its own CMake library, so they can share like 98% of their codebase.

Happily, once I did all this, the test worked fine on both devices I tested.

Impact of changes

• EMAC test works again, so we have a way to verify that ethernet works

Migration actions required

Documentation

Updated https://github.com/mbed-ce/mbed-os/wiki/Running-the-Mbed-Greentea-Tests-Locally

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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)

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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

I verified that the EMAC test passes on both LPC1768 and NUCLEO_F429ZI, using a K64F board as the CTP tester.

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