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/electron/modules/calendar_spec.js

@@ -7,7 +7,7 @@ describe("Calendar module", () => {
 	 */
 	const doTest = async (cssClass) => {
 		let elem = await helpers.getElement(`.calendar .module-content .event${cssClass}`);
-		expect(await elem.isVisible()).toBe(true);
+		await expect(elem.isVisible()).resolves.toBe(true);
 	};
 
 	afterEach(async () => {

tests/electron/modules/compliments_spec.js

@@ -8,7 +8,7 @@ describe("Compliments module", () => {
 	const doTest = async (complimentsArray) => {
 		await helpers.getElement(".compliments");
 		const elem = await helpers.getElement(".module-content");
-		expect(elem).not.toBe(null);
+		expect(elem).not.toBeNull();
 		expect(complimentsArray).toContain(await elem.textContent());
 	};