raid6/test: replace echo -e with printf

-e is a non-standard echo option, echo output is
implementation-dependent when it is used. Replace echo -e with printf as
suggested by POSIX echo manual.

Cc: NeilBrown <neilb@suse.de>
Cc: Jim Kukunas <james.t.kukunas@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Acked-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>