Add support for testing with multiple users/browsers

/**
 * Open a new browser tab, after saving the authenticated user from the current tab (so that we can restore it when
 * switching back).
 *
 * @param setup the test setup
 * @return the handle of the new tab
 */
protected String openNewBrowserTab(TestUtils setup)
{
    // Save the authenticated user from the current tab before moving to the new tab in order to be able to restore
    // it later when we switch back to the current tab.
    saveAuthenticatedUser(setup);
    return setup.getDriver().switchTo().newWindow(WindowType.TAB).getWindowHandle();
}

/**
 * Switch to another (existing) browser tab, restoring its authenticated user (that was saved when the tab was
 * left).
 *
 * @param setup the test setup
 * @param tabHandle the tab to switch to
 */
protected void switchToBrowserTab(TestUtils setup, String tabHandle)
{
    // Save the authenticated user from the current tab before moving to the specified tab in order to be able to
    // restore it later when we switch back to the current tab.
    saveAuthenticatedUser(setup);
    setup.getDriver().switchTo().window(tabHandle);
    restoreAuthenticatedUser(setup);
}

private void saveAuthenticatedUser(TestUtils setup)
{
    WebDriver driver = setup.getDriver();
    String currentTabHandle = driver.getWindowHandle();
    this.cookiesByTab.put(currentTabHandle, driver.manage().getCookies());
    this.secretTokenByTab.put(currentTabHandle, setup.getSecretToken());
}

private void restoreAuthenticatedUser(TestUtils setup)
{
    WebDriver driver = setup.getDriver();
    String currentTabHandle = driver.getWindowHandle();
    driver.manage().deleteAllCookies();
    this.cookiesByTab.get(currentTabHandle).forEach(cookie -> driver.manage().addCookie(cookie));
    setup.setSecretToken(this.secretTokenByTab.get(currentTabHandle));
}

@Test
@Order(1)
void multiUser(TestUtils setup, TestReference testReference) throws Exception
{
    setup.loginAsSuperAdmin();
    setup.rest().savePage(testReference, "{{velocity}}XXX: $xcontext.userReference.name{{/velocity}}", "xwiki/2.1",
        "", "");
    ViewPage viewPage = setup.gotoPage(testReference);
    assertEquals("XXX: superadmin", viewPage.getContent());

    String secondTabHandle = openNewBrowserTab(setup);
    setup.createUserAndLogin("alice", "pass");
    viewPage = setup.gotoPage(testReference);
    assertEquals("XXX: alice", viewPage.getContent());

    switchToBrowserTab(setup, firstTabHandle);
    viewPage = setup.gotoPage(testReference);
    assertEquals("XXX: superadmin", viewPage.getContent());

    switchToBrowserTab(setup, secondTabHandle);
    viewPage = setup.gotoPage(testReference);
    assertEquals("XXX: alice", viewPage.getContent());
}