fix: rewrite .js to .ts imports in test files (re-apply)

Re-apply .ts import rewrites to test files after they were lost
in a previous git checkout operation. Also fix bare package
specifiers that had erroneous .ts extension.

Assisted-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bennypowers

packages/test-runner-commands/test/send-mouse/sendMousePlugin.test.ts

@@ -1,6 +1,6 @@
 import path from 'path';
 import selenium from 'selenium-standalone';
-import { runTests } from '@web/test-runner-core/test-helpers.js';
+import { runTests } from '@web/test-runner-core/test-helpers.ts';
 import { chromeLauncher } from '@web/test-runner-chrome';
 import { webdriverLauncher } from '@web/test-runner-webdriver';
 import { playwrightLauncher } from '@web/test-runner-playwright';

packages/test-runner-junit-reporter/test/junitReporter.test.ts

@@ -5,7 +5,7 @@ import globby from 'globby';
 
 import { chromeLauncher } from '@web/test-runner-chrome';
 import type { TestRunnerCoreConfig } from '@web/test-runner-core';
-import { runTests } from '@web/test-runner-core/test-helpers.js';
+import { runTests } from '@web/test-runner-core/test-helpers.ts';
 import { junitReporter } from '../src/junitReporter.ts';
 
 const __dirname = import.meta.dirname;