Fix flakey time-senstive group filter specs

See merge request gitlab-org/gitlab-ce!19880

Render calendar feed inline when accessed from GitLab

Closes #47672

See merge request gitlab-org/gitlab-ce!19698

With text/calendar as Content-Type, the browser always downloads the
content as a file (even ignoring the Content-Disposition header). We
want to display the content inline when accessed from GitLab, similarly
to the RSS feed.

Resolve "Long labels names result in overflowing their description block under the Labels page"

Closes #47915

See merge request gitlab-org/gitlab-ce!19868

Fix flakey time-sensitive move slash command with unauthorized project

Closes #47936

See merge request gitlab-org/gitlab-ce!19793

Fix flakey time-sensitive remove deploy key spec

See merge request gitlab-org/gitlab-ce!19800

Fix flakey time-sensitive user/assignee autocomplete dropdown

See merge request gitlab-org/gitlab-ce!19799

This passed previously because the filtered group search is debounced by 0.5s. The test cleared the input, entered `group1`, cleared the input, entered nothing, and the all of groups are still listed because of the 0.5s debounce hasn't triggered and the test passes before anything is actually filtered.

Even if we assert that the list is filtered before clearing the input, the test still fails because the nature of the `fill_in 'filter', with: ""` method is that ["if you're setting the value to "", no keys are ever actually sent, hence no [`input`] event"](https://github.com/teamcapybara/capybara/issues/203#issuecomment-557281) and we never filter back to everything in the list.

So the solution is two-fold, add in the assertions that the list is actually filtered after each step. Then use a method that fills the input with proper events fired.