tst_checkpoint_signal_child: implemented timeout

If a child exits before opening TST_CHECKPOINT_FIFO
for reading, tst_checkpoint_signal_child() issued from the parent
will block forever.

To handle such situations added timeout logic to tst_checkpoint_signal_child();

Signed-off-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@oracle.com>
Reviewed-by: Jan Stancek <jstancek@redhat.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>
add test for tst_checkpoint_signal_child timing out

This is simple test for patch which adds timeout to
tst_checkpoint_signal_child().

The test is expected to end with:

  Failed to open fifo 'tst_checkpoint_fifo' at
  tst_checkpoint_no_child.c:43: errno=ETIMEDOUT(110): Connection timed out

Signed-off-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Cyril Hrubis <chrubis@suse.cz>