Add browser_tests README (#553)

browser_tests/README.md

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+# Playwright Testing for ComfyUI_frontend
+
+This document outlines the setup and usage of Playwright for testing the ComfyUI_frontend project.
+
+## Setup
+
+Ensure you have Node.js v20 or later installed. Then, set up the Chromium test driver:
+
+```bash
+npx playwright install chromium --with-deps
+```
+
+## Running Tests
+
+There are two ways to run the tests:
+
+1. **Headless mode with report generation:**
+   ```bash
+   npx playwright test
+   ```
+   This runs all tests without a visible browser and generates a comprehensive test report.
+
+2. **UI mode for interactive testing:**
+   ```bash
+   npx playwright test --ui
+   ```
+   This opens a user interface where you can select specific tests to run and inspect the test execution timeline.
+
+   ![Playwright UI Mode](https://github.com/user-attachments/assets/6a1ebef0-90eb-4157-8694-f5ee94d03755)
+
+## Screenshot Expectations
+
+Due to variations in system font rendering, screenshot expectations are platform-specific. Please note:
+
+- We maintain Linux screenshot expectations as our GitHub Action runner operates in a Linux environment.
+- To set new test expectations:
+  1. Create a pull request from a `Comfy-Org/ComfyUI_frontend` branch.
+  2. Add the `New Browser Test Expectation` tag to your pull request.
+  3. This will trigger a GitHub action to update the screenshot expectations automatically.
+
+> **Note:** If you're making a pull request from a forked repository, the GitHub action won't be able to commit updated screenshot expectations directly to your PR branch.