WebDriver: auto-install pytest instead of importing it from wpt tools directory

WebDriver: auto-install pytest instead of importing it from wpt tools directory
https://bugs.webkit.org/show_bug.cgi?id=180243

Reviewed by Brian Burg.

Tools:

We don't really need the (old) version included in wpt tools dir, so we can simply remove it and use autoinstall
instead.

* Scripts/webkitpy/thirdparty/__init__.py:
(AutoinstallImportHook.find_module): Check pytest.
(AutoinstallImportHook._install_pytest): Install pytest.
* Scripts/webkitpy/webdriver_tests/webdriver_selenium_executor.py: Import autoinstalled pytest.
* Scripts/webkitpy/webdriver_tests/webdriver_test_runner_w3c.py:
(WebDriverTestRunnerW3C.run): Update the subtest path since the new pytest uses a different strategy for
rootdir.
* Scripts/webkitpy/webdriver_tests/webdriver_w3c_executor.py: Import autoinstalled pytest.

WebDriverTests:

* imported/selenium/py/conftest.py: Stop patching this to use yield_fixture, new pytest supports this.
* imported/selenium/py/setup.cfg: Stop patching this, since pytest supports tool:pytest as group name.
* imported/selenium/py/test/selenium/webdriver/common/alerts_tests.py: Stop patching this to use yield_fixture,
new pytest supports this.
* imported/selenium/py/test/selenium/webdriver/common/cookie_tests.py: Ditto.
* imported/selenium/py/test/selenium/webdriver/common/frame_switching_tests.py: Ditto.
* imported/selenium/py/test/selenium/webdriver/common/page_load_timeout_tests.py: Ditto.
* imported/selenium/py/test/selenium/webdriver/common/window_switching_tests.py: Ditto.
* imported/selenium/py/test/selenium/webdriver/safari/conftest.py: Ditto.
* imported/selenium/py/test/selenium/webdriver/support/event_firing_webdriver_tests.py: Ditto.
* imported/w3c/importer.json: Stop importing pytest.
* imported/w3c/pytest.ini: Added.
* imported/w3c/tools/pytest/: Removed.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@225447 268f45cc-cd09-0410-ab3c-d52691b4dbfc