Move WTR concurrency config out of groups field. (...)

Items in `groups` are not able to override all fields of the outer config
object, only a very limited subset.[^1]

[^1]: https://github.com/modernweb-dev/web/blob/6c5893cc79c91e82f9aec35d3011c6e33ce878cc/packages/test-runner-core/src/config/TestRunnerGroupConfig.ts

Author: bicknellr

packages/tests/web-test-runner.config.js

@@ -138,6 +138,8 @@ module.exports = {
   files: ['custom-elements/html/**/*.test.(js|html)'],
   rootDir: '../..',
   nodeResolve: true,
+  concurrency: 1,
+  concurrentBrowsers: 1,
   plugins: [compileJSPlugin()],
   groups: [
     {
@@ -150,8 +152,6 @@ module.exports = {
     },
     {
       name: 'sauce',
-      concurrency: 1,
-      concurrentBrowsers: 1,
       // This is intentionally a getter so that browser launchers are only
       // created on demand.
       get browsers() {