In PR #18574 setting `window.uiManager` was moved into the `src` folder
to avoid intermittent integration test failures because at the time we
lacked a way to register event listeners early (before PDF.js loads).
However, in PR #18617 this functionality got introduced, so we can now
use the new way of setting up the event bus in the tests to move this
back to the `test` folder again and to reduce the amount of test-only
code in the main codebase as discussed in PR #18574.
Partially reverts e037c5711d3d2413669e9b6c275986adf24a295b.
db06011032