Test coverage for src/components/app

In trying to find our worst offenders for test coverage, [src/components/app/](https://codecov.io/gh/devtools-html/perf.html/tree/dde68013d994ec113c7513f85f2b0eadee9af2cf/src/components/app) stands out as being particularly bad. We might need to take on [enzyme](https://github.com/airbnb/enzyme) as a dependency and shallow rendering to properly deal with some of these components.

Files ordered by lines uncovered:

* [ ] Root.js (77 lines uncovered)
* [ ] ProfileSharing.js (67 lines uncovered)
* [ ] ProfileViewer.js (40 lines uncovered)
* [ ] UrlManager.js (32 lines uncovered)
* [ ] Home.js (27 lines uncovered)
* [ ] SymbolicationStatusOverlay.js (23 lines uncovered)
* [ ] TabBar.js (17 lines uncovered)
* [ ] ProfileFilterNavigator.js (9 lines uncovered)
* [ ] FooterLinks.js (3 lines uncovered)
* [ ] Initializing.js (2 lines uncovered)

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-692)