LU-14816 tests: mark sanity test_230d SLOW

LU-14816 tests: mark sanity test_230d SLOW

Running sanity test_230d takes an average 15 minutes to finish,
and up to an hour in some cases, but has almost never failed.
Move it over to run only with SLOW=yes.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I02ec35c1533a6a97b5400d4419664b43ab49c502
Reviewed-on: https://review.whamcloud.com/44616
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>