liblog: test: do not LOG in signal handler

(cherry pick from commit 50af7f8b)

- We actually are logging in a signal handler, the title is bluster
  to remind developers. It is not a reliable path though as it can
  lock up. Our goal is to minimize the chances of a lockup regardless
  in the name of stability only. The test remains to catch regression
  in the name of code quality and reliability. Expected to be
  >99.999% reliable.
- Add a new _correct_ duplicate test that uses signal to release a
  semaphore to a thread that performs the task. This path is expected
  to be 100% reliable.

Bug: 27405083
Change-Id: Ibb7cf4b13e34ebfac2db2af8724b7db7a27f81a8