build: run flaky tests in Travis

Skipping flaky tests in CI is an anti-pattern that should be avoided,
as we do in our ownCI. Failing flaky tests don’t need to be blockers
for a green CI result, but they should be run and reported *somehow*.

PR-URL: https://github.com/nodejs/node/pull/27158


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>