add `closeExtraTabs` to browser

[kellyselden]

so you can clean up new tabs in an afterEach, without closing the main tab if there was a test error and it didn't create a second tab.

something like

async function closeExtraTabs() {
  while (await this.browser._browser.getWindowHandles() > 1) {
    await this.browser.closeTab();
  }
}