Suggestions to avoid corner cases

[igormcoelho]

Congratulations for this nice example and for Github Education! I'm using a lot!
As explained in the README, some cases may be hard for students... so, perhaps it's better to assume a "clean before test", also to verify that "hello.cpp" has changed (or not), and also also to reinforce deletion of "a.out" to prevent strange errors when it doesn't exist.
Finally, a good thing is to add a ".gitignore" removing "a.out". Since it's an artifact, it's good for students (and for GitHub) to prevent storing it as binary.

I have some other suggestions, that could be put on README perhaps, that I learned on practice. When creating the test cases with Catch, it's useful to add grades to each test separately, such as: "./a.out Test1", "./a.out Test2", etc.
Maybe it's trivial, but I took some days of exploration to finally understand that Catch was able to do such filtering.. which is complementary to simply grading the whole exercise with "make test".

Anyway, feel free to discard or change this if necessary. Regards!