LU-18662 tests: skip fstrim on unsupported devices

LU-18662 tests: skip fstrim on unsupported devices

if an underlying device doesn't support fstrim, then never try it
again.

Fixes: 6872cf9a36 ("LU-17722 tests: trim tmpfs from wait_delete_completed()")
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: Ie7e49800ed0161c968e453a531b9701f3459a318
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57851
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>