LU-12125 tests: allow racer to specify extra tasks

LU-12125 tests: allow racer to specify extra tasks

Add the RACER_EXTRA environment variable to allow racer.sh to run
extra tasks.

Test-Parameters: trivial testlist=racer env=DURATION=3600,RACER_EXTRA=file_rename:10,dir_create:10,dir_remote:10,dir_migrate:3
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ic810a248a2dd665a163e0efea8c9af0e4461e09b
Reviewed-on: https://review.whamcloud.com/41231
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Elena Gryaznova <c17455@cray.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>