LU-13352 tests: fix subtest duration for ONLY_REPEAT

LU-13352 tests: fix subtest duration for ONLY_REPEAT

The subtest duration printed when the test passes was the total
elapsed time rather than the per-iteration time.  Print the
per-subtest time at the end of each iteration.

Fixes: e16e3d46ee8c ("LU-13169 tests: add ONLY_REPEAT parameter to repeat subtests")
Test-Parameters: trivial testlist=sanity env=ONLY=76,ONLY_REPEAT=10
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I0758df03ceb12c3c7c8d9cd62561d4ee7b3ebbe5
Reviewed-on: https://review.whamcloud.com/37872
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Wei Liu <sarah@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>