LU-10070 tests: Fix replay-single test_85b

test_85b of replay single sets a default striping on $DIR
and does not remove it.  This makes it impossible to
correctly test self-extending layouts, so fix this first.

cray-bug-id: LUS-2528
Signed-off-by: Patrick Farrell <paf@cray.com>
Change-Id: I0057c8403e3dae2437cf0c8810af8086e2971c35
Reviewed-on: https://review.whamcloud.com/33777
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Ben Evans <bevans@cray.com>
Reviewed-by: Vitaly Fertman <c17818@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>