use es6 imports in tests

src/function/__tests__/combine.test.js

@@ -1,6 +1,6 @@
-export default test => {
-    const combine = require('../index').combine;
+import combine from '../combine';
 
+export default test => {
     test.ok(typeof combine === 'function', 'combine should be a function');
 
     const a = val => val * 10;

src/object/__tests__/filter.test.js

@@ -1,6 +1,6 @@
-export default test => {
-    const filter = require('../index').filter;
+import filter from '../filter';
 
+export default test => {
     test.ok(typeof filter === 'function', 'filter should be a function');
     test.done();
 };