raid6: test: make sure all intermediate and artifact files are .gitignored

Currently when the raid6test utility is built, the resulting binary and
an int.uc file are not being ignored, which can get inadvertently
committed as a result when one works on the raid6 code. Ignore them to
make `git status` clean at all times.

Signed-off-by: WANG Xuerui <git@xen0n.name>
Link: https://lore.kernel.org/r/20230731104911.411964-5-kernel@xen0n.name
Signed-off-by: Song Liu <song@kernel.org>