Implement tests using Playwright (#68)

* Set up Playwright and add first test

* Add npm run commands for running tests
Run tests in github actions

* Fix tests on non Mac environments

* Add method to HeynoteEditor class to set the buffer content

* Add more tests

* Set Github action job name

tests/test-utils.js

@@ -0,0 +1,44 @@
+import { test, expect } from '@playwright/test';
+
+export function pageErrorGetter(page) {
+    let messages = [];
+    page.on('pageerror', (error) => {
+        messages.push(`[${error.name}] ${error.message}`);
+    });
+    return () => messages;
+}
+
+
+export class HeynotePage {
+    constructor(page) {
+        this.page = page
+        this.getErrors = pageErrorGetter(page)
+        this.isMac = process.platform === "darwin"
+    }
+
+    async goto() {
+        await this.page.goto("/")
+        await expect(this.page).toHaveTitle(/Heynote/)
+        expect(this.getErrors()).toStrictEqual([])
+    }
+
+    async getBlocks() {
+        return await this.page.evaluate(() => window._heynote_editor.getBlocks())
+    }
+
+    async getContent() {
+        return await this.page.evaluate(() => window._heynote_editor.getContent())
+    }
+    
+    async setContent(content) {
+        await this.page.evaluate((content) => window._heynote_editor.setContent(content), content)
+    }
+
+    async getBlockContent(blockIndex) {
+        const blocks = await this.getBlocks()
+        const content = await this.getContent()
+        expect(blocks.length).toBeGreaterThan(blockIndex)
+        const block = blocks[blockIndex]
+        return content.slice(block.content.from, block.content.to)
+    }
+}