tests: facilitate background process debug in ceph-helpers.sh

tests: facilitate background process debug in ceph-helpers.sh

When displaying the output of a background process, do it on stderr so
that it is not bufferized. Otherwise the output of the background
process may be displayed after it completed.

Prefix the output of a background process with the PID of the process
known to the parent instead of the PID of the awk process processing the
output. When wait_background loops, it will print the process on which
it is waiting and it is confusing that they do not match with the PID
prefixing the process output.

Refs: http://tracker.ceph.com/issues/17830

Signed-off-by: Loic Dachary <loic@dachary.org>