Travis: New test to check for eslint rules

[dereksmart]

Any errors reported by eslint (yarn lint) will now fail Travis in this new test suite.

To test:

• Verify that tests pass
• You should still see the warnings in the test suite in the travis UI

[eliorivero]

This test continues failing

1) WP_Test_Jetpack_Sync_Sender::test_limits_execution_time_of_do_sync

but it's not the scope of this PR. The changes introduced work fine 🐑

[oskosk]

LGTM. Tested running yarn lint and npm run lint after introducing a lodash named import and checking for build errors in PR #6368 created only for my tests of this PR . Errors were shown. when lint run locally and and Travis failed on #6368