Add a readme to unit tests

Author: paul-szczepanek-arm

UNITTESTS/README.md

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+## Stubs, mocks and fakes in Mbed OS
+
+To make unit testing easier, Mbed OS provides some ready-made CMake libraries you can add in your `target_link_libraries`. 
+Libraries can be object files that contain stubs, mocks and fakes but also sets of include paths - this way you can gain access to normal headers from Mbed OS.
+
+All the libraries available are in the `CMakeLists.txt` files in `mbed-os/UNITTESTS/stubs` and `mbed-os/UNITTESTS/fakes`.
+
+The most common ones you might need are `mbed-os-fakes-ble` and `mbed-os-fakes-event-queue`.
+
+### Fakes
+
+While stubs are self explanatory and don't offer anything beyond empty implementations, fakes allow for more complex unittests that can simulate whole subsystems.
+
+#### mbed-os-fakes-ble
+
+This library provides a fake BLE implementation that uses mocks instead of real BLE components for `Gap`, `GattServer`, `GattClient`, `SecurityManager`.
+
+There is no need to initialise a fake BLE instance; it is ready to go and can be used just like a normal BLE instance:
+
+```c++
+BLE *ble = &BLE::Instance();
+```
+
+This call also initialises mocks. 
+Do no cache the BLE instance pointer, or pointer to `GAP`, `GattServer` etc. between tests. 
+You must get the instance fresh at the start of the test.
+
+You can retrieve all the BLE APIs from the instance just like with a normal one:
+
+```c++
+Gap &gap = ble->gap();
+GattClient &client = ble->gattClient();
+GattServer &server = ble->gattServer();
+SecurityManager &sm = ble->securityManager();
+```
+
+Whenever an API call is made, the implementation will be called. 
+These are replaced in the fake BLE with google mocks.
+This means you can set expectations on them. 
+
+```c++
+EXPECT_CALL(ble::gap_mock(), reset());
+```
+
+This will set up an expectation that at some point during the test the `Gap::reset` method will be called.
+
+The way GoogleTest works means that if you set expectations on your mocks they must be destroyed at the end of each test. 
+This is done through the fake BLE instance special method:
+
+```c++
+ble::delete_mocks();
+```
+
+#### mbed-os-fakes-event-queue
+
+This is a fake event queue that doesn't bring in any dependencies from mbed-os. 
+Its API is simplified and it only offers limited functionality.
+
+If you choose to use it you must not also include a library that brings in real headers for the event queue as they would conflict.
+
+The API calls supported are for simple calls `call`, `call_in` and the `cancel` method.
+
+The event queue is not run in real time and must be progressed manually. 
+You may use `dispatch(int milliseconds)` and `dispatch_forever()` to process events in the queue. 
+This way you can simulate the passage of time in your test.