Add web tests to the CI

[Ellpeck]

Since we use the newer defineConfig system for extension tests (over the old runTests system), we can't support web tests yet. Notably, the config for web tests says:

/**
 * Configuration that runs in browsers.
 * @todo: this is incomplete, and does not yet function
 */
export interface IWebTestConfiguration extends IBaseTestConfiguration {
    platform: 'firefox' | 'webkit' | 'chromium';
}

Once this works, we can change our test defineConfig to return an array instead, where one of the entries uses the chromium platform.