tests: hot fix, ignore 'unused' warning

Those structures should be used in the future.

Signed-off-by: Zhouqi Jiang <luojia@hust.edu.cn>

tests/build-rename.rs

@@ -136,6 +136,7 @@ impl rustsbi::EnvInfo for DummyEnvInfo {
     }
 }
 
+#[allow(unused)] // FIXME: hot fix, use it on unit test in the future.
 struct RealEnvInfo;
 
 impl rustsbi::EnvInfo for RealEnvInfo {

tests/forward-struct.rs

@@ -2,6 +2,7 @@ use rustsbi::{Forward, RustSBI};
 
 // The `Forward` structure must build
 
+#[allow(unused)] // FIXME: hot fix, use it on unit test in the future.
 #[derive(RustSBI)]
 struct ForwardAll {
     #[rustsbi(