refactor common test helpers into test/

This way we can share them with multiple tests

test/env.js

@@ -0,0 +1,15 @@
+const Environment = require('jest-environment-jsdom');
+
+/**
+ * A custom environment to set the TextEncoder
+ * Thanks https://stackoverflow.com/a/57713960/1976323
+ */
+module.exports = class CustomTestEnvironment extends Environment {
+    async setup() {
+        await super.setup();
+        if (typeof TextEncoder === 'undefined') {
+            const { TextEncoder } = require('util');
+            this.global.TextEncoder = TextEncoder;
+        }
+    }
+};