test_firmware: add test case for SIGCHLD on sync fallback

It has been reported that SIGCHLD will trigger an immediate abort
on sync firmware requests which rely on the sysfs interface for a
trigger. This is unexpected behaviour, this reproduces this issue.

This test case currenty fails.

Reported-by: Martin Fuzzey <mfuzzey@parkeon.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>