Come closer and check out all the nice playgrounds for your training pleasure found in those nifty sub directories. Each even contains its own README telling you more about it!
For your own training area, place a new subfolder in the project, which ideally contains a README.md detailling the task and a solutions directory containing the solutions to the task. Lastly, just add
$(OUTDIR)/your-folder-test: your-folder/source.c your-folder/source2.c your-folder/test.c
to the Makefile below the other entries of that schema (Do include the -test, otherwise we hit weirdnesses with make). A test.c file, checking whether the task has successfully been solved, is greatly appreciated as well.