CMake: improve target dependencies

[agarny]

Right now, if I want to build libCellML and run the test, I need to do something like:

mkdir build
cd build
cmake -G Ninja ..
ninja
ninja test

while it would be nice if I could simply call ninja test rather than first ninja and then ninja test. In fact, it would be nice if we could build any target without first having to call ninja or something else.