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/modules/weather_forecast_spec.js

@@ -24,7 +24,7 @@ describe("Weather module: Weather Forecast", () => {
 		for (const [index, icon] of icons.entries()) {
 			it(`should render icon ${icon}`, async () => {
 				const elem = await helpers.waitForElement(`.weather table.small tr:nth-child(${index + 1}) td:nth-child(2) span.wi-${icon}`);
-				expect(elem).not.toBe(null);
+				expect(elem).not.toBeNull();
 			});
 		}
 
@@ -46,7 +46,7 @@ describe("Weather module: Weather Forecast", () => {
 		for (const [index, opacity] of opacities.entries()) {
 			it(`should render fading of rows with opacity=${opacity}`, async () => {
 				const elem = await helpers.waitForElement(`.weather table.small tr:nth-child(${index + 1})`);
-				expect(elem).not.toBe(null);
+				expect(elem).not.toBeNull();
 				expect(elem.outerHTML).toContain(`<tr style="opacity: ${opacity};">`);
 			});
 		}
@@ -72,14 +72,14 @@ describe("Weather module: Weather Forecast", () => {
 
 		it("should render custom table class", async () => {
 			const elem = await helpers.waitForElement(".weather table.myTableClass");
-			expect(elem).not.toBe(null);
+			expect(elem).not.toBeNull();
 		});
 
 		it("should render colored rows", async () => {
 			const table = await helpers.waitForElement(".weather table.myTableClass");
-			expect(table).not.toBe(null);
-			expect(table.rows).not.toBe(null);
-			expect(table.rows.length).toBe(5);
+			expect(table).not.toBeNull();
+			expect(table.rows).not.toBeNull();
+			expect(table.rows).toHaveLength(5);
 		});
 
 		const precipitations = [undefined, "2.51 mm"];