LU-8465 e2fsck: fix f_multithread_ok test

LU-8465 e2fsck: fix f_multithread_ok test

Don't use $OUT for both the input amd output of a pipeline,
as the output file is truncated befor the input is read.

Fix the handling in the failure case to generate the
*.failed file, and print the actual $test_name instead
of "test_name".

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I66513a0422a0e70fd7a887b215e3a4e6c9c250e5
Reviewed-on: https://review.whamcloud.com/40068
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@whamcloud.com>