Adjust test and code locations to match (UI)

[neiljp]

Description of suggested feature

I just closed #422, which as a move/split PR was long outdated and partially replaced by already-merged changes.

However, at some point it would likely still be useful to make changes such as:

• move view element tests (eg. StreamsView, MessagesView) from ui/test_ui_tools into ui_tools/test_views (matching existing code positions)
• split popup code out from ui_tools/views into ui_tools/popups (matching existing test split)

This has been discussed previously, and would help the developer experience in terms of locating matching code to tests and vice versa, but we help off at times due to conflicting open PRs.

In the past we have often started new work in a new file, and later moved code or tests around to join it.

[zulipbot]

Hello @zulip/server-refactoring members, this issue was labeled with the "area: refactoring" label, so you may want to check it out!