refactor(mock-consensus): 🔐 remove Keyring

previously, we added a `Keyring` newtype. this was a low-effort, thin
newtype wrapper around a `BTreeMap<VerificationKey, SigningKey>`.

this commit removes it.

NB: this is based on #4001.

crates/test/mock-consensus/src/builder.rs

@@ -6,8 +6,9 @@
 mod init_chain;
 
 use {
-    crate::{keyring::Keyring, TestNode},
+    crate::{Keyring, TestNode},
     bytes::Bytes,
+    std::collections::BTreeMap,
 };
 
 /// A buider, used to prepare and instantiate a new [`TestNode`].
@@ -57,9 +58,15 @@ impl Builder {
             );
         }
 
-        Self {
-            keyring: Keyring::new_with_size(1),
-            ..self
-        }
+        // Generate a consensus key.
+        let sk = ed25519_consensus::SigningKey::new(rand_core::OsRng);
+        let vk = sk.verification_key();
+        tracing::trace!(verification_key = ?vk, "generated consensus key");
+
+        // Place it into the keyring.
+        let mut keyring = BTreeMap::new();
+        keyring.insert(vk, sk);
+
+        Self { keyring, ..self }
     }
 }

crates/test/mock-consensus/src/lib.rs

@@ -4,9 +4,13 @@
 //
 //  see penumbra-zone/penumbra#3588.
 
+use {
+    ed25519_consensus::{SigningKey, VerificationKey},
+    std::collections::BTreeMap,
+};
+
 pub mod block;
 pub mod builder;
-pub mod keyring;
 
 mod abci;
 
@@ -23,9 +27,14 @@ pub struct TestNode<C> {
     consensus: C,
     last_app_hash: Vec<u8>,
     height: tendermint::block::Height,
-    keyring: self::keyring::Keyring,
+    keyring: Keyring,
 }
 
+/// An ordered map of consensus keys.
+///
+/// Entries in this keyring consist of a [`VerificationKey`] and a [`SigningKey`].
+type Keyring = BTreeMap<VerificationKey, SigningKey>;
+
 impl<C> TestNode<C> {
     pub const CHAIN_ID: &'static str = "penumbra-test-chain";
 
@@ -40,6 +49,16 @@ impl<C> TestNode<C> {
         // - https://doc.rust-lang.org/std/fmt/#formatting-traits
         format!("{:02X?}", self.last_app_hash)
     }
+
+    /// Returns a reference to the test node's set of consensus keys.
+    pub fn keyring(&self) -> &Keyring {
+        &self.keyring
+    }
+
+    /// Returns a mutable reference to the test node's set of consensus keys.
+    pub fn keyring_mut(&mut self) -> &mut Keyring {
+        &mut self.keyring
+    }
 }
 
 impl<C> TestNode<C>