ui_session: return airgun session as a context manager

[pnovotny]

Problem Statement

If a UI test is using the new session approach via target_sat.ui_session(), the browser window is not closed at the end of the test. Also the screenshot is not taken if the test fails.

Solution

Satellite.ui_session() method needs to return the airgun Session as a context manager, so the __exit__ method is called, which handles screenshot capture and closing the browser window.