tests: posix: eventfd: Add ioctl F_SETFL test

Add a test to protect against future regressions in the ioctl F_SETFL
operation of eventfd. Flags are set and unset and validity of the file
descriptor is checked by reading and writing.

Signed-off-by: Celina Sophie Kalus <hello@celinakalus.de>
(cherry picked from commit 325f22a1)