LU-14773 tests: quiet down some verbose messages

LU-14773 tests: quiet down some verbose messages

Don't print anything into the test logs for normal background
operations that are run as part of run_one(), so that they
don't clutter the test output with repeated/useless messages.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ib6a49fc268e4cd0ad92c71a391865ce2d73ebbe5
Reviewed-on: https://review.whamcloud.com/44034
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Elena Gryaznova <elena.gryaznova@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>