[core, node] Use es5 syntax for imports in expressions tests

Instead of forcing ES6 module syntax, at this point, it is less
distruptive to fix expression tests to use ES5 import syntax.

platform/node/test/expression.test.js

@@ -1,6 +1,6 @@
-import {run} from '../../../mapbox-gl-js/test/integration/lib/expression';
-import mbgl from '../index';
-import ignores from './ignores.json';
+const {run} = require('../../../mapbox-gl-js/test/integration/lib/expression');
+const mbgl = require('../index');
+const ignores = require('./ignores.json');
 
 let tests;