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*.

.travis.yml

@@ -65,4 +65,4 @@ jobs:
         - make -j2 > /dev/null
         - make -j1 build-addons build-js-native-api-tests build-node-api-tests > /dev/null
       script:
-        - JOBS=2 FLAKY_TESTS=skip make -s -j1 V= test-ci | grep -F -e "---" -e "..." -v
+        - JOBS=2 FLAKY_TESTS=dontcare make -s -j1 V= test-ci | grep -F -e "---" -e "..." -v