CI: setup HTTPS in web-test-runner for BrowserStack tests

To have tests work Browserstack Safari (also below iOS 15), as the tests are run in an iframe,
rewriting localhost as hostname, making WebCrypto not available.

We keep HTTP for the non-browserstack tests so that in local testing,
generating self-signed certs is not required.

test/web-test-runner.browserstack.config.js

@@ -0,0 +1,49 @@
+import { browserstackLauncher } from '@web/test-runner-browserstack';
+import wtrConfig from './web-test-runner.config.js';
+
+const sharedBrowserstackCapabilities = {
+  'browserstack.user': process.env.BROWSERSTACK_USERNAME,
+  'browserstack.key': process.env.BROWSERSTACK_ACCESS_KEY,
+
+  project: `openpgpjs/${process.env.GITHUB_EVENT_NAME || 'push'}${process.env.LIGHTWEIGHT ? '/lightweight' : ''}`,
+  name: process.env.GITHUB_WORKFLOW || 'local',
+  build: process.env.GITHUB_SHA || 'local',
+  'browserstack.acceptInsecureCerts': true
+};
+
+export default {
+  ...wtrConfig,
+  protocol: 'https:',
+  http2: true,
+  sslKey: './127.0.0.1-key.pem',
+  sslCert: './127.0.0.1.pem',
+  testsStartTimeout: 45000,
+  browserStartTimeout: 120000,
+  testsFinishTimeout: 450000,
+  concurrentBrowsers: 3,
+  concurrency: 1, // see https://github.com/modernweb-dev/web/issues/2706
+  coverage: false,
+  groups: [], // overwrite the field coming from `wrtConfig`
+  browsers: [
+    browserstackLauncher({
+      capabilities: {
+        ...sharedBrowserstackCapabilities,
+        browserName: 'Safari iOS 14',
+        device: 'iPhone 12',
+        real_mobile: true,
+        os: 'ios',
+        os_version: '14' // min supported version (iOS/Safari < 14 does not support native BigInts)
+      }
+    }),
+    browserstackLauncher({
+      capabilities: {
+        ...sharedBrowserstackCapabilities,
+        browserName: 'Safari iOS latest',
+        device: 'iPhone 16',
+        real_mobile: true,
+        os: 'ios',
+        os_version: 'latest'
+      }
+    })
+  ]
+};