raid6: test: cosmetic cleanups for the test Makefile

Use tabs/spaces consistently: hard tabs for marking recipe lines only,
spaces for everything else.

Also, the OPTFLAGS declaration actually included the tabs preceding the
line comment, making compiler invocation lines unnecessarily long. As
the entire block of declarations are meant for ad-hoc customization
(otherwise they would probably make use of `?=` instead of `=`), move
the "Adjust as desired" comment above the block too to fix the long
invocation lines.

Signed-off-by: WANG Xuerui <git@xen0n.name>
Link: https://lore.kernel.org/r/20230731104911.411964-4-kernel@xen0n.name
Signed-off-by: Song Liu <song@kernel.org>