tests: test_pidfile.sh lingering processes

tests: test_pidfile.sh lingering processes

For the mon, using ps aux to find the process will only show part of the
command line and the $RUNID may be missing. For the OSD, the $RUNID is
passed using an environment variable and will not show at all.

There is no need to test both ceph-osd and ceph-mon since they both end
up in the same code path, pidfile_write. Add a log line that only shows
when the --pid-file is empty and grep for it in the simplest possible
way.

The TEST_pidfile test failed to kill a process because there was
no pidfile with its pid. This is fixed by backuping the pidfile before
it is modified.

http://tracker.ceph.com/issues/14834 Fixes: #14834

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