Reuse nop closure in DatabaseEloquentBuildertest.php

Continued from https://github.com/laravel/framework/pull/2729

HHVM reuses closures if they have the same body, causing DatabaseEloquentBuilderTest::testEagerLoadRelationsLoadTopLevelRelationships() to fail. Instead, save the closures to variables and use those.
Add back missing rparen
Merge pull request #2759 from alexmalyshev/master

Reuse nop closure in DatabaseEloquentBuildertest.php