LU-15140 tests: cleanup of recovery-*-scale tests fails

LU-15140 tests: cleanup of recovery-*-scale tests fails

Bash trap handler is executed only after completition of
current command, so under big I/O load it can be executed
after test and cleanup phase finished.

Run I/O load in background overcome bash limitation.

Lustre-commit: f252abc6690247ee9608dbde80238add0ecaed8c
Lustre-change: https://review.whamcloud.com/45824

Test-Parameters: trivial clientcount=6 mdtcount=2 mdscount=2 osscount=2 austeroptions=-R failover=true iscsi=1 env=FAILOVER_PERIOD=180 testlist=recovery-double-scale env=SLOW=yes
Test-Parameters: clientcount=5 mdtcount=2 mdscount=2 osscount=2 austeroptions=-R failover=true iscsi=1 env=FAILOVER_PERIOD=180 env=DURATION=82800 testlist=recovery-mds-scale env=SLOW=yes
Test-Parameters: clientcount=5 mdtcount=2 mdscount=2 osscount=2 austeroptions=-R failover=true iscsi=1 env=DURATION=82800 testlist=recovery-random-scale env=SLOW=yes

Signed-off-by: Elena Gryaznova <elena.gryaznova@hpe.com>
HPE-bug-id: LUS-2649
Change-Id: I3c91cac4d3f9af9863e8f48ba8a6bae02190ccb4
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/46749
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>