CI: pass the number of retries to mocha

[fflorent]

The CI may fail for random reasons (mostly due to timeouts).

It becomes hard to know whether the failure is due to our additions or to the instabilities.

Mocha allows to globally set a number of retries for tests: https://mochajs.org/api/mocha#retries

May be worth for nbrowser tests (AFAIK, these are the most unstable). WDYT?

[paulfitz]

I've been wanting to look at what the tests are doing, during the obvious and common timeouts. Is it somehow downloading some big silly webdriver binary, or what is going on? Ideally someone would take some time to understand what exactly is timing out before wrapping it in retries.