Write a test for dragging range selections

I believe this regressed previously before too.

See the STR from #1845.

Ideally this test should be written a bit higher up the component hierarchy, probably using the `` component itself, `src/components/timeline/index.js`. It should use mouse events and the react testing library to simulate a user dragging the selection.

This test will go in: src/test/components/Timeline.test.js

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-133)