Enable and apply ESLint Jest rules (#3270)

Jest was in the plugin array of the ESLint configuration, but no rules
were enabled. So ESLint hasn't checked any Jest rules yet.

So I activated the recommended Jest rules and added a few more. Then I
fixed the issues (mostly automatically). I have deactivated the rules
"jest/expect-expect" and "jest/no-done-callback" for the time being, as
they would have entailed major changes. I didn't want to make the PR too
big.

I'm not a Jest expert, but the changes so far look good to me. What do
you think of that @khassel? 🙂

tests/e2e/env_spec.js

@@ -21,7 +21,7 @@ describe("App environment", () => {
 
 	it("should show the title MagicMirror²", async () => {
 		const elem = await helpers.waitForElement("title");
-		expect(elem).not.toBe(null);
+		expect(elem).not.toBeNull();
 		expect(elem.textContent).toBe("MagicMirror²");
 	});
 });