Clean up our tests in preparation for the testing sprint (#2466)

* Fix coverage reporting

Codecov hasn't been working recently because Vitest doesn't report coverage by default.

* Suppress some noisy log lines

Closes https://github.com/element-hq/element-call/issues/686

* Store test files alongside source files

This way we benefit from not having to maintain the same directory structure twice, and our linters etc. will actually lint test files by default.

* Stop using Vitest globals

Vitest provides globals primarily to make the transition from Jest more smooth. But importing its functions explicitly is considered a better pattern, and we have so few tests right now that it's trivial to migrate them all.

* Remove Storybook directory

We no longer use Storybook.

* Configure Codecov

Add a coverage gate for all new changes and disable its comments.

* upgrade vitest

---------

Co-authored-by: Timo <toger5@hotmail.de>

codecov.yaml

@@ -0,0 +1,13 @@
+# Don't post comments on PRs; they're noisy and the same information can be
+# gotten through the checks section at the bottom of the PR anyways
+comment: false
+coverage:
+  status:
+    project:
+      default:
+        # Track the impact of changes on overall coverage without blocking PRs
+        informational: true
+    patch:
+      default:
+        # Expect 80% coverage on all lines that a PR touches
+        target: 80%