Add a command-line flag to print status updates while tests are running.

**Description**
Right now Catch2 prints nothing until all tests have completed. It would be helpful if there was some easy way to have it print a line each time it is starting a new test case so that I can see whether the tests are making progress. (Also to keep me entertained enough to avoid alt-tabbing to a browser and losing 15 minutes. 🙄) I'm thinking something nice and simple like:

`Starting test (7/135): NAME_OF_TEST`

You could even have it start with `\r` and not end with `\n` so that it only takes up a single terminal line, like ninja does when the output `isatty`