rename WebDriverTestBase::getSession() and WebDriverTestBase::assertSession() to be less confusing

WebDriverTestBase::getSession() and WebDriverTestBase::assertSession() do completely different things, despite having very similar names:

- assertSession -- returns \Drupal\Tests\WebAssert A new web-assert option for asserting the presence of elements with.

- getSession -- returns \Behat\Mink\Session The active Mink session object.

The two methods return completely different things, despite sounding like maybe one checks the session is OK and the other getting you the actual session to work with. Also, something called assertFoo() looks like a PHPUnit assertion, and you wouldn't expect that to return anything.

Could these be renamed to something clearer?