Verify (and fix) search onboarding tour

Created by: fkling

The search onboarding popover wasn't visible in a Percy screenshot in https://github.com/sourcegraph/sourcegraph/pull/36044, which seems to indicate that there issues with the integration of the tour and CodeMirror.

search-onboarding.test.ts should be adapted once #36044 was merged and any issues surfaced by this should be fixed.