tests: Mark the new test as unstable, since it seems to fail in 3.8.
tests: Mark the new test as unstable, since it seems to fail in 3.8.

And I don't want nor have the time to debug it right now.

rust-lang/rust-bindgen,emilio/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen