aead: test fixups (#1881)

- ~~There's no reason for these tests to be gated on `alloc`~~ (aah, we
need it, but can go finer-grained)
- Fixes comment on object safety / dyn compatibility test for `Aead`
- Adds dyn compatibility test for `AeadInOut`

Author: tarcieri

aead/src/lib.rs

@@ -431,12 +431,16 @@ impl<const N: usize> Buffer for heapless::Vec<u8, N> {
     }
 }
 
-#[cfg(feature = "alloc")]
 #[cfg(test)]
 mod tests {
     use super::*;
 
-    /// Ensure that `AeadInPlace` is object-safe
+    /// Ensure that `Aead` is `dyn`-compatible
+    #[cfg(feature = "alloc")]
+    #[allow(dead_code)]
+    type DynAead<N, T> = dyn Aead<NonceSize = N, TagSize = T>;
+
+    /// Ensure that `AeadInOut` is `dyn`-compatible
     #[allow(dead_code)]
-    type DynAeadInPlace<N, T> = dyn Aead<NonceSize = N, TagSize = T>;
+    type DynAeadInOut<N, T> = dyn AeadInOut<NonceSize = N, TagSize = T>;
 }