Add a tests summary/ tests run stats

[SuperKogito]

The link provided by maelle in urlstechie/urlchecker-action#61 gave me an idea, so I am just laying this out here to get your input on it. So far we only provide urls checks/tests results, but wouldn't it be also nice to provide a summary of the tests and some stats, something like:

Urls checks summary
-----------------------------
Total count of tested files     : 10
Total count of tested urls      : 51
Total count of working urls     : 48  
Total count of failed urls      : 3

etc.

[vsoch]

This would be a nice little tweak to have at the print at the end of the file. But without checks, where would this go? The output file currently handles printing passed and failed, and (I just realized) we need to add the whitelisted set too (which are currently ignored). I'll open an issue for this now.