init/epoll_test: Improve this test

Add a move constructor in the CatchDtor class. Check the .emplace()
result. Simplify the destructor. Initialize handler_invoked. Explain the
purpose of this test.

Bug: 213617178
Change-Id: I4d6f97dbb2705a2f2dd78e449ae8de74e90b102f
Signed-off-by: Bart Van Assche <bvanassche@google.com>