My first e2e test for option page

e2e/options.test.js

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+const express = require('express');
+const lanthan = require('lanthan');
+const path = require('path');
+const assert = require('assert');
+const eventually = require('./eventually');
+
+describe("options page", () => {
+  let firefox;
+  let session;
+  let browser;
+
+  before(async() => {
+    firefox = await lanthan.firefox({
+      spy: path.join(__dirname, '..'),
+      builderf: (builder) => {
+        builder.addFile('build/settings.js');
+        builder.addFile('build/settings.html');
+      },
+    });
+    await firefox.session.installAddonFromPath(path.join(__dirname, '..'));
+    session = firefox.session;
+    browser = firefox.browser;
+  });
+
+  after(async() => {
+    if (firefox) {
+      await firefox.close();
+    }
+  });
+
+  beforeEach(async() => {
+    let tabs = await browser.tabs.query({});
+    for (let tab of tabs.slice(1)) {
+      await browser.tabs.remove(tab.id);
+    }
+  })
+
+  it('saves current config on blur', async () => {
+    let url = await browser.runtime.getURL("build/settings.html")
+    await session.navigateTo(url);
+
+    let textarea = await session.findElementByCSS('textarea');
+    const updateTextarea = async(value) => {
+      await session.executeScript(`document.querySelector('textarea').value = '${value}'`)
+      await textarea.sendKeys(' ');
+      await session.executeScript(() => document.querySelector('textarea').blur());
+    }
+
+    await updateTextarea(`{ "blacklist": [ "https://example.com" ] }`);
+
+    let { settings } = await browser.storage.local.get('settings');
+    assert.equal(settings.source, 'json')
+    assert.equal(settings.json, '{ "blacklist": [ "https://example.com" ] } ')
+
+    await updateTextarea(`invalid json`);
+
+    settings = (await browser.storage.local.get('settings')).settings;
+    assert.equal(settings.source, 'json')
+    assert.equal(settings.json, '{ "blacklist": [ "https://example.com" ] } ')
+
+    let error = await session.findElementByCSS('.settings-ui-input-error');
+    let text = await error.getText();
+    assert.ok(text.startsWith('SyntaxError:'))
+  });
+});
+