erasure-code,test: silence -Wunused-variable warnings

erasure-code,test: silence -Wunused-variable warnings

GCC 7.3 does not support [[maybe_unused]] very well, so it emits
-Wunused-variable warnings even if the variables are marked with
maybe_unused. moreover, the C++17 standard does not forbid these
warnings:

> For an entity marked maybe_unused, implementations are encouraged not
to emit a warning that the entity is unused, or that the entity is used
despite the presence of the attribute.

see also n4659, dcl.attr.unused

so, in this change, the warnings are silenced manually.

Signed-off-by: Kefu Chai <kchai@redhat.com>