Fix headless chrome timeout error in grunt tests

[gnikonorov]

The JS tests run by grunt will occasionally fail with the error below. We should fix it. The error is sporadic and goes away on rerun ( usually )

Run npm test

> @ test /home/runner/work/pytest-html/pytest-html
> grunt test

Running "qunit:src" (qunit) task
Testing testing/js_test_report.html >> There was an error with headless chrome
Fatal error: Navigation timeout of 5000 ms exceeded�
npm ERR! Test failed.  See above for more details.
Error: Process completed with exit code 1.

Run link: https://github.com/pytest-dev/pytest-html/pull/405/checks?check_run_id=1525098011