Evaluate adding a test to prevent "`web-ext run` exits while watching a directory that includes a broken link" from regressing

[rpl]

In #2159, one of our dependencies (likely watchpack) was internally rejecting a promise which was not handled and in future nodejs versions any unhandled rejection triggers a fatal error and the nodejs process does exit.

If I'm not mistaken that behavior is the default starting from nodejs >= v15 (and it can be explicitly enabled at least from nodejs >= v12 using the --unhandled-rejections=strict cli option).

#2159 has been fixed by the dependency that was triggering it and it is not reproducible anymore on web-ext v6.0.0 (it was still reproducible in web-ext v5.5.0), but it may be reasonable to add an explicit test (maybe an integration test) to make sure that the CI job would catch it if it does regress again in the future.