Makefile: add "make check" option to run unit tests for given TARGET

[rfairley]

Describe the feature
make check TARGET=F4 builds and run the unit tests for the F4 board. F7 can also be specified as a target. This would involve invoking the mingw gcc/g++ compilers from the makefile and returning the gtest test output to the terminal. If possible, we should try and check for the test build dependencies, and return a helpful error message if some are not found e.g. "g++ not found, install mingw".

Reason for request
This would be another step towards unifying the RobotTest project into Robot, I think. It'd be convenient to invoke this from the command line, without needing to add a unit test run configuration in Eclipse. This would make the run configuration for unit tests simpler, as well, by invoking make check TARGET=... (possibly we can version control the unit tests run configuration as well?). Possibly, make check could be a step in the CI pipeline and it'd log results for each board.

Timeline
No particular timeline.

[gokuldharan]

I'm working on this right now!