Chore: fix failing cli test (#15041)

Author: mdjermanovic

tests/fixtures/rules/wrong/custom-rule.js

@@ -1,5 +1,3 @@
 module.exports = function() {
-
-    "use strict";
-    return (null).something;
+    throw new Error("Boom!");
 };

tests/lib/cli.js

@@ -512,7 +512,7 @@ describe("cli", () => {
                 const exit = await cli.execute(code);
 
                 assert.strictEqual(exit, 2);
-            }, /Error while loading rule 'custom-rule': Cannot read property/u);
+            }, /Error while loading rule 'custom-rule': Boom!/u);
         });
 
         it("should return a warning when rule is matched", async () => {