the temporary directory holding the log file collecting the scylla
subprocess's output is specified by the test itself, and it is
`test_tempdir`. but unfortunately, cql-pytest/run.py is not aware
of this. so `cleanup_all()` is not able to print out the logging
messages at exit. as, please note, cql-pytest/run.py always
collect "log" file under the directory created using `pid_to_dir()`
where pid is the spawned subprocesses. but `object_store/run` uses
the main process's pid for its reusable tempdir.
so, with this change, we also register a cleanup func to printout
the logging message when the test exits.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
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