tevent: Regression test to ensure that a tevent backend can cope with separate read...

tevent: Regression test to ensure that a tevent backend can cope with separate read/write events on a single fd.

This tests the multiplex fd changes to the epoll backend to
ensure they work correctly.

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>