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
Our build system does support both canonical forms for running tests, `ctest` and `make test` which would be the first  point of contact, the same as for any other project.