fix(tests): stub system class for command checkRoot

With 9786477 the order commands were run in was changed so the check wasn't properly run. By stubbing the system class instead of the ui class, we restore original behavior

lib/command.js

@@ -136,10 +136,7 @@ class Command {
                 ui.log(`Unable to use "${dir}" (error ${err}). Please fix the issue and try again.`, 'red', true);
                 process.exit(1);
             }
-            /* istanbul ignore next */
-            // In order to keep the rest of the function from running, chdir / exit is stubbed to
-            // throw an error in the tests so this line is technically unreachable. Since it's only
-            // a debug statement, we're not going to worry about it
+
             debug('Finished updating directory');
         }
 

test/unit/command-spec.js

@@ -218,15 +218,15 @@ describe('Unit: Command', function () {
             }
             const Command = proxyquire(modulePath, {
                 './utils/check-root-user': checkRootUserStub,
-                './ui': ShortCircuit
+                './system': ShortCircuit
             });
 
             const TestCommand = class extends Command {};
             TestCommand.global = true;
             TestCommand.allowRoot = true;
 
             try {
-                TestCommand._run('test', {});
+                TestCommand._run('test', {dir: '.'});
             } catch (e) {
                 expect(e).to.exist;
                 expect(e.message).to.equal('you shall not pass');