tests: posix: eventfd_basic: add new test

Introduce new test for eventfd, which will test it in only basic
scope. The real reason why this is introduced is to have a test with
minimal set of configuration dependencies, such as disabled
pthreads (compared to full tests/posix/eventfd/ variant). This allows to
test eventfd mechanism with both POSIX_API=n and POSIX_API=y.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>