Fix name resolution errors related to cfg(test)

This commit is for fixing the below error. `cfg(test)` does not seem
to be passed to the imported library. To sidestep this issue, the
duplicate `MockDevice` is maintained (one for `io`'s tests and another
for `macro`'s tests).

```
$ ./scripts/tests/crates.sh
error[E0432]: unresolved import `io::test`
  --> rmm/src/macro.rs:77:13
   |
77 |     use io::test::MockDevice;
   |             ^^^^ could not find `test` in `io`
```

Signed-off-by: Changho Choi <[email protected]>

lib/io/src/lib.rs

@@ -55,7 +55,7 @@ pub fn stdout() -> SpinlockGuard<'static, Stdout> {
 #[cfg(test)]
 pub mod test {
     extern crate alloc;
-    use crate::io::{ConsoleWriter, Device, Result, Stdout, Write};
+    use super::{ConsoleWriter, Device, Result, Stdout, Write};
     use alloc::boxed::Box;
     use alloc::string::String;
     use alloc::vec::Vec;

rmm/src/macro.rs

@@ -74,12 +74,57 @@ macro_rules! const_assert_size {
 
 #[cfg(test)]
 mod test {
-    use crate::io::test::MockDevice;
-    use crate::io::{stdout, Write as IoWrite};
+    extern crate alloc;
+
     use crate::{eprintln, println};
     use alloc::boxed::Box;
+    use alloc::string::String;
+    use alloc::vec::Vec;
+    use core::cell::RefCell;
+    use io::{stdout, Write as IoWrite};
+    use io::{ConsoleWriter, Device, Result, Write};
+
+    pub struct MockDevice {
+        buffer: RefCell<Vec<u8>>,
+        ready: bool,
+    }
 
-    extern crate alloc;
+    impl MockDevice {
+        pub const fn new() -> Self {
+            MockDevice {
+                buffer: RefCell::new(Vec::new()),
+                ready: false,
+            }
+        }
+
+        pub fn output(&self) -> String {
+            String::from_utf8(self.buffer.borrow().to_vec()).unwrap()
+        }
+
+        pub fn clear(&mut self) {
+            self.buffer.borrow_mut().clear()
+        }
+    }
+
+    impl Device for MockDevice {
+        fn initialize(&mut self) -> Result<()> {
+            self.ready = true;
+            Ok(())
+        }
+
+        fn initialized(&self) -> bool {
+            self.ready
+        }
+    }
+
+    impl Write for MockDevice {
+        fn write_all(&mut self, buf: &[u8]) -> Result<()> {
+            self.buffer.borrow_mut().extend_from_slice(buf);
+            Ok(())
+        }
+    }
+
+    impl ConsoleWriter for MockDevice {}
 
     #[test]
     fn println_without_arg() {