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/unit/global_vars/root_path_spec.js

@@ -14,11 +14,11 @@ describe("'global.root_path' set in js/app.js", () => {
 	});
 
 	it("should not modify global.root_path for testing", () => {
-		expect(global.root_path).toBe(undefined);
+		expect(global.root_path).toBeUndefined();
 	});
 
 	it("should not modify global.version for testing", () => {
-		expect(global.version).toBe(undefined);
+		expect(global.version).toBeUndefined();
 	});
 
 	it("should expect the global.version equals package.json file", () => {