re-enable flaky test: "Completing a negated filter should insert the filter with - prefix (codemirror6)"

Created by: mrnugget

I'm going to disable this test in a PR. Here is the ticket to re-enable after flakiness is fixed.

Example failures:

• https://buildkite.com/sourcegraph/sourcegraph/builds/159870#0181f1be-764f-4fe4-92b6-5f83376ee5dc
• https://buildkite.com/sourcegraph/sourcegraph/builds/159856#0181f1a7-0398-4598-b76b-14ee05a05b7e
• https://buildkite.com/sourcegraph/sourcegraph/builds/159836#0181f15a-cd4d-49a8-9380-b1c64428bf0d