LU-13730 tests: add file mirroring to racer

LU-13730 tests: add file mirroring to racer

Add "lfs mirror extend" to racer to add mirrors to existing files.

Test-Parameters: trivial testlist=racer,racer,racer
Test-Parameters: fstype=zfs testlist=racer,racer,racer
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Iaa64ed2de54533838ce955f88a1be592923ebbe5
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/41368
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Zhenyu Xu <bobijam@hotmail.com>
Reviewed-by: Alex Deiter <alex.deiter@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>