Add test information to contributing guidelines

It turns out that we didn't have any information about how to run tests in the contributing guidelines.  Unfortunately, new contributors may not know how to run the tests (https://github.com/libgit2/libgit2/pull/2649).

This adds some quick start information to help new contributors.

Also includes changes from #2403.

/cc @swisspol
Regarding `make test`, I don't want to muddy the issues with telling people who are generating Makefiles with Cmake to do one thing and telling people who are generating Xcode or Visual Studio projects to do something else.

I also think that simply running the clar binary directly is superior to `ctest` since the latter won't easily take a list of tests to run.