JS tests fail often in CI

Example: https://drone.owncloud.com/owncloud/contacts/871/8/6

START:
22 06 2021 01:16:04.546:INFO [karma-server]: Karma v4.4.1 server started at http://0.0.0.0:9876/
22 06 2021 01:16:04.548:INFO [launcher]: Launching browsers firefox_custom with concurrency unlimited
22 06 2021 01:16:04.552:INFO [launcher]: Starting browser firefox via Remote WebDriver
22 06 2021 01:16:04.694:ERROR [WebDriver]: WebDriver command failed { spec:
   { platform: 'Linux',
     testName: 'Karma test',
     tags: [],
     version: '',
     base: 'WebDriver',
     browserName: 'firefox' },
  error:
   { Error: getaddrinfo ENOTFOUND selenium selenium:4444
       at GetAddrInfoReqWrap.onlookup [as oncomplete] (dns.js:56:26)
     errno: 'ENOTFOUND',
     code: 'ENOTFOUND',
     syscall: 'getaddrinfo',
     hostname: 'selenium',
     host: 'selenium',
     port: '4444' } }
22 06 2021 01:16:04.700:INFO [WebDriver]: Killed Karma test.
22 06 2021 01:16:04.700:ERROR [launcher]: Cannot start firefox via Remote WebDriver
    undefined
22 06 2021 01:16:04.700:ERROR [launcher]: firefox via Remote WebDriver stdout: 
22 06 2021 01:16:04.700:ERROR [launcher]: firefox via Remote WebDriver stderr:

But the test command exists with success status. So the step does not "fail" in CI.

The next step fails "coverage-cahce" https://drone.owncloud.com/owncloud/contacts/871/8/7 time="2021-06-22T01:16:07Z" level=error msg="Could not match files" error="file does not exist"

The coverage output does not exist, so we notice the fail at that step.

See if we can do something to:

make the JS test command exit with "fail" when it can't start the browser...
make the browser startup more reliable

owncloud / contacts

JS tests fail often in CI #716