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/newsfeed_spec.js

@@ -13,20 +13,20 @@ describe("Newsfeed module", () => {
 
 		it("should show the newsfeed title", async () => {
 			const elem = await helpers.waitForElement(".newsfeed .newsfeed-source");
-			expect(elem).not.toBe(null);
+			expect(elem).not.toBeNull();
 			expect(elem.textContent).toContain("Rodrigo Ramirez Blog");
 		});
 
 		it("should show the newsfeed article", async () => {
 			const elem = await helpers.waitForElement(".newsfeed .newsfeed-title");
-			expect(elem).not.toBe(null);
+			expect(elem).not.toBeNull();
 			expect(elem.textContent).toContain("QPanel");
 		});
 
 		it("should NOT show the newsfeed description", async () => {
 			await helpers.waitForElement(".newsfeed");
 			const elem = document.querySelector(".newsfeed .newsfeed-desc");
-			expect(elem).toBe(null);
+			expect(elem).toBeNull();
 		});
 	});
 
@@ -38,14 +38,14 @@ describe("Newsfeed module", () => {
 
 		it("should not show articles with prohibited words", async () => {
 			const elem = await helpers.waitForElement(".newsfeed .newsfeed-title");
-			expect(elem).not.toBe(null);
+			expect(elem).not.toBeNull();
 			expect(elem.textContent).toContain("Problema VirtualBox");
 		});
 
 		it("should show the newsfeed description", async () => {
 			const elem = await helpers.waitForElement(".newsfeed .newsfeed-desc");
-			expect(elem).not.toBe(null);
-			expect(elem.textContent.length).not.toBe(0);
+			expect(elem).not.toBeNull();
+			expect(elem.textContent).not.toHaveLength(0);
 		});
 	});
 
@@ -57,7 +57,7 @@ describe("Newsfeed module", () => {
 
 		it("should show malformed url warning", async () => {
 			const elem = await helpers.waitForElement(".newsfeed .small", "No news at the moment.");
-			expect(elem).not.toBe(null);
+			expect(elem).not.toBeNull();
 			expect(elem.textContent).toContain("Error in the Newsfeed module. Malformed url.");
 		});
 	});
@@ -70,7 +70,7 @@ describe("Newsfeed module", () => {
 
 		it("should show empty items info message", async () => {
 			const elem = await helpers.waitForElement(".newsfeed .small");
-			expect(elem).not.toBe(null);
+			expect(elem).not.toBeNull();
 			expect(elem.textContent).toContain("No news at the moment.");
 		});
 	});