LU-4684 tests: enable racer directory migration

LU-4684 tests: enable racer directory migration

Enable the dir_migrate test by default in racer test runs.

Update test selection logic to match newer script code style.

Lustre-change: https://review.whamcloud.com/41359
Lustre-commit: TBD (from 84cbbc51ef6c04fd62bc32a5f31dd2a2a2f47ebb)

Test-Parameters: trivial testlist=racer env=DURATION=3600
Test-Parameters: fstype=zfs testlist=racer env=DURATION=600
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ifba84c64b30d90b4a159232751b68c48c88dafcc
Reviewed-on: https://review.whamcloud.com/41963
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>