Silence core-js warnings during tests (#661)

Co-authored-by: Ward Peeters <ward@coding-tech.com>

Author: developit

test/fixtures/custom-babelrc/.babelrc

@@ -6,6 +6,7 @@
 				"loose": true,
 				"modules": false,
 				"useBuiltIns": "usage",
+				"corejs": 2,
 				"targets": {
 					"esmodules": true
 				}