Add getHighchartsChart test helper

ember-highcharts/rollup.config.mjs

@@ -20,7 +20,7 @@ export default {
     // up your addon's public API. Also make sure your package.json#exports
     // is aligned to the config here.
     // See https://github.com/embroider-build/embroider/blob/main/docs/v2-faq.md#how-can-i-define-the-public-exports-of-my-addon
-    addon.publicEntrypoints(["index.js", "**/*.js"]),
+    addon.publicEntrypoints(['index.js', '**/*.js']),
 
     // These are the modules that should get reexported into the traditional
     // "app" tree. Things in here should also be in publicEntrypoints above, but
@@ -30,7 +30,6 @@ export default {
       'helpers/**/*.js',
       'modifiers/**/*.js',
       'services/**/*.js',
-      "modifiers/**/*.js"
     ]),
 
     // Follow the V2 Addon rules about dependencies. Your code can import from

ember-highcharts/src/test-support/helpers.ts

@@ -0,0 +1,30 @@
+import { find, waitFor, waitUntil } from '@ember/test-helpers';
+
+import Highcharts from 'highcharts';
+
+type ChartContainerElement = {
+  dataset: {
+    highchartsChart: string;
+  };
+};
+
+/**
+ * Finds the Highcharts chart container for a given chart and returns the Highcharts.Chart for it.
+ * This is useful for testing that the options for the chart are what you expect.
+ * @param selector The selector for your chart wrapper element
+ */
+export async function getHighchartsChart(selector: string) {
+  await waitFor(`${selector} .chart-container`);
+
+  const chartContainer = find(
+    `${selector} .chart-container`,
+  ) as unknown as ChartContainerElement;
+
+  await waitUntil(
+    () => Highcharts.charts[Number(chartContainer.dataset.highchartsChart)],
+  );
+
+  return Highcharts.charts[
+    Number(chartContainer.dataset.highchartsChart)
+  ] as Highcharts.Chart;
+}