Build: fix test failure on Node 11 (#11100)

This is a workaround for tschaub/mock-fs#256. It seems like we might eventually need to find an alternative to mock-fs for future Node versions, but this workaround should make the build pass in the meantime.

tests/lib/util/npm-utils.js

@@ -162,8 +162,7 @@ describe("npmUtils", () => {
         });
 
         it("should return false if package.json does not exist", () => {
-            mockFs({});
-            assert.strictEqual(npmUtils.checkPackageJson(), false);
+            assert.strictEqual(npmUtils.checkPackageJson("/"), false);
         });
     });