Tests headless context destruction on session close.

Creating context {disposeOnDetach: true} in session1
Creating context {disposeOnDetach: true} in session1
Creating context {disposeOnDetach: true} in session1
Number of contexts created: 3
Creating context {disposeOnDetach: false} in session2
Creating context {disposeOnDetach: false} in session2
Number of contexts created: 5
Detaching from session1
Number of contexts created: 2
Detaching from session2
Number of contexts created: 2