Add tests for rendering to offscreen canvas (#7175)

Add tests for using an offscreen canvas for Chart.js. These tests are
almost identical to existing tests, but with offscreen canvas enabled.

Co-authored-by: David Winegar <david.winegar@getcruise.com>

test/BasicChartWebWorker.js

@@ -0,0 +1,26 @@
+// This file is a basic example of using a chart inside a web worker.
+// All it creates a new chart from a transferred OffscreenCanvas and then assert that the correct platform type was
+// used.
+
+// Receives messages with data of type: { type: 'initialize', canvas: OffscreenCanvas }
+// Sends messages with data of types: { type: 'success' } | { type: 'error', errorMessage: string }
+
+import Chart from '../src';
+
+onmessage = function(event) {
+	try {
+		const {type, canvas} = event.data;
+		if (type !== 'initialize') {
+			throw new Error('invalid message type received by worker: ' + type);
+		}
+
+		const chart = new Chart(canvas);
+		if (!(chart.platform instanceof Chart.platforms.BasicPlatform)) {
+			throw new Error('did not use basic platform for chart in web worker');
+		}
+
+		postMessage({type: 'success'});
+	} catch (error) {
+		postMessage({type: 'error', errorMessage: error.stack});
+	}
+};