rename some missed tests files

Author: zloirock

tests/pure/esnext.array.at.js renamed to tests/pure/es.array.at.js

@@ -1,6 +1,6 @@
 import { STRICT } from '../helpers/constants';
 
-import at from 'core-js-pure/features/array/at';
+import at from 'core-js-pure/es/array/at';
 
 QUnit.test('Array#at', assert => {
   assert.isFunction(at);

tests/pure/esnext.string.at-alternative.js renamed to tests/pure/es.string.at-alternative.js

@@ -1,8 +1,8 @@
 // TODO: disabled by default because of the conflict with another proposal
 import { STRICT } from '../helpers/constants';
 
-import at from 'core-js-pure/features/string/at';
-import Symbol from 'core-js-pure/features/symbol';
+import at from 'core-js-pure/es/string/at';
+import Symbol from 'core-js-pure/es/symbol';
 
 QUnit.skip('String#at', assert => {
   assert.isFunction(at);