Simplify how the render functions are defined and used in test files

[julienw]

Before this patch, we have 2 different render functions: render and renderWithRouter. We also have a StoreProvider that's just react-redux's Provider pre-configured, that we use in some hooks-related test files.

The 2 render functions are "created" in beforeEach and exposed by setupTests. Instead of "creating" them, in this patch they're now defined directly, and they can be imported directly from test-utils in the test files. The goal is that it's now easier to create new flavors for these functions, that I'd like to use in #582.

The store object is imported from setupTests instead of being passed as a parameter. I considered creating it in the render functions (possibly as a default parameter, so that we could pass another store if needed) and returning it, but this would require changing all call sites, so I decided to keep it simple for now. We could always change it more later.

This is on top of #584 and #586 to minimize rebase conflicts, so take care to only look at the few un-labeled commits at the end of the commit list.

[netlify[bot]]

✅ Deploy Preview for mozilla-perfcompare ready!

Name	Link
🔨 Latest commit	1f79b48b42cbf1f50084d48d0a47049ff380540d
🔍 Latest deploy log	https://app.netlify.com/sites/mozilla-perfcompare/deploys/65a50667cdc58a000878f500
😎 Deploy Preview	https://deploy-preview-587--mozilla-perfcompare.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (3d0654a) 99.34% compared to head (1f79b48) 99.34%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## beta #587 +/- ## ======================================= Coverage 99.34% 99.34% ======================================= Files 62 62 Lines 1366 1366 Branches 189 189 ======================================= Hits 1357 1357 Misses 9 9 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.