Unit tests compilation is unnecessarily slow

[carllocos]

Currently, it takes 4 to 5 minutes to fully compile all the unit test files whereas the actual running of each test is blazing fast.

I think this is because, for each unit test file, we rebuild the whole Warduino project. Building WARDuino as a shared library across each unit test file should increase performance. This just requires a change in the CMakeLists.txt file.