Investigate web-ext lint functional test intermittency on travis windows workers

[rpl]

As part of #1505 we are going to re-run the functional tests if they fail once in a travis windows worker.

The reason for the timeout is likely due to the IO needed to load all the addons-linter dependencies, but we need to investigate it a bit more to determine if there is a better way to workaround/fix this issue (which only seems to happen on travis, I've never been able to see the test failing on a local windows system).

As a side note: In b65226a we have raised the mocha timeout when running in a windows worker: https://github.com/mozilla/web-ext/blob/a9b9a76b047cbb4b8313bb364f4094788184e859/tasks/configs/mochaTest.js#L3-L5

Unfortunately it have not been enough, as the test was still failing intermittently on travis windows workers from time to time, but it has got even worst after the last update of the addons-linter dependency.