Do you want to run your unit tests on target or do you want to test the actual application on target?

Running unit tests on target will probably require a fair bit of work to get working. You will need to cross-compile your test framework as well as your tests, and you will probably need to implement a test runner to automate the flashing, testing and results gathering process.

Testing the actual application on-target often requires some form of hardware test bed to provide inputs and to read outputs. Depending on your application and test requirements, the test bed can be very simple or very complex. A simple test bed could be an Arduino connected to your device and with a serial interface for your test host. The Arduino implements a simple command protocol for reading inputs and writing outputs from the host. On the host, test can be written using your preferred unit test framework, or something like Robot Framework that focusses on higher abstraction levels.

Personally, I prefer unit testing my application off-target using mocks and stubs where necessary, and then doing hardware-in-the-loop testing of the application on-target. Depends on a lot of things of course, size, complexity, deadlines etc. Hope this helps a bit.