Add integration test to verify shutdown behavior

This test needs to use maven-invoker-plugin to run in a separate JVM,
because ImageJ1 calls System.exit(0) upon successful shutdown. We really
want to verify that indeed this action occurs.

Note that maven-surefire-plugin and maven-failsafe-plugin do not support
System.exit getting called from tests, so using maven-invoker-plugin is
the best way to go here.