refactor tests and babel config. Fixes target browsers.

This is basically the change propagated from the current
default preact tempalate. Now the code compiles according to set
browsers.

.babelrc

@@ -1,5 +1,9 @@
 {
-    "presets": [ 
+  "env": {
+    "test": {
+      "presets": [
         ["preact-cli/babel", { "modules": "commonjs" }]
-    ]
+      ]
+    }
+  }
 }