LU-16384 tests: dump lustre log if DEBUG_RMMOD set

LU-16384 tests: dump lustre log if DEBUG_RMMOD set

just to simplify local development and use existing code in
lustre_rmmod script:
DEBUG_RMMOD=<logfile> sh sanity.sh will dump a text lustre log to <logfile>.
it can be DEBUG_RMMOD=- to direct lustre log to standard output.

Test-Parameters: trivial
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I8d72e1e9cecb354bcc5d41ab3cca5767a298c668
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/49374
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>