fix flaky Ruby test `element_spec.rb` (#16709)

* fix flaky Ruby test `element_spec.rb`

Wait until the new URL gets loaded.

Sometimes after executing `driver.navigate.to url_for('new_page.html')` the browser is still showing the previous page.

Example of failure:

```ruby
Selenium::WebDriver::Element raises if different element receives click
     Failure/Error: element = wait_for_element(id: 'contents', timeout: 10)

     Selenium::WebDriver::Error::TimeoutError:
       could not find element {:id=>"contents", :timeout=>10} in 25 seconds;
       page url: http://localhost:49887/resultPage.html?x=25&y=25;
       page source:
       <p id="greeting">Success!</p> ...

     # ./rb/lib/selenium/webdriver/common/wait.rb:76:in `until'
     # /Users/runner/work/selenium/selenium/rb/spec/integration/selenium/webdriver/spec_support/helpers.rb:93:in `wait_for_element'
     # ./rb/spec/integration/selenium/webdriver/element_spec.rb:35:in `block (2 levels) in <module:WebDriver>'
```

See https://github.com/SeleniumHQ/selenium/actions/runs/20055175531/job/57520193372#step:19:920 for example.

* Open a blank page before every test

It's a good practice to avoid tests affecting each other:
1. Open "about:blank" page - this stops any current activity / background requests / animations on the previous page
2. Open an empty page which clears sessionStorage, localStorage and cookies.

This technique allows reusing the browser between tests, while keeping the tests independent.