LU-16878 tests: use RUNAS_UID / RUNAS_GID for NRS TBF

LU-16878 tests: use RUNAS_UID / RUNAS_GID for NRS TBF

Some of the sanityn NRS TBF test hardcode the use of uid 500 and gid
500. They are not guaranteed to exist so use RUNAS_UID and RUNAS_GID
instead.

Test-Parameters: trivial testlist=sanityn env=ONLY=77
Change-Id: Ie987c70e94918c5cddadb632a4a3a3caac12c96f
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/51238
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alex Deiter <alex.deiter@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>