runner: prevent Test::Unit from running in at_exit

Test::Unit is finding and running 0 tests in tork-runner(1) itself.
tork-engine sometimes sent :idle event prematurely
keep sub-Rails connected to our in-memory database

When another Rails instance is started by Capybara, it sets up a new
connection to the test database instead of re-using our in-memory DB.

http://www.spacevatican.org/2012/8/18/threading-the-rat/
tork: forgotten docs: ENTER by itself does nothing