linux: test that the return code, in the error case, has the right sign

tests/test.rs

@@ -10,6 +10,17 @@
 #[macro_use]
 extern crate sc;
 
+#[cfg(target_os = "linux")]
+#[test]
+fn ebadf() {
+    static MESSAGE: &'static str = "Hello, world!";
+
+    unsafe {
+        assert_eq!(syscall!(WRITE, 4, MESSAGE.as_ptr(), MESSAGE.len()) as isize,
+                   -9)
+    }
+}
+
 // getpid() is POSIX but that doesn't guarantee it's a system call.
 #[cfg(any(target_os = "linux", target_os = "freebsd"))]
 #[test]