So that it's compiled only once and not for every test. This change
reduced recompilation time for a dummy test module from 40 seconds to
4 on my laptop.
This change also makes it now possible to pass seastar framework command
line arguments to every test using this framework.
If the engine exits premauterly the _task exchanger may be occupied by
a pending task in which case give() will block indefinitely. Fix by
using interruption.
