sign: move TestSigningBackend to lib

We need to make `TestSigningBackend` available for cli tests, as we are
implementing the `jj sign` command in following changes.

lib/src/lib.rs

@@ -75,7 +75,6 @@ pub mod repo_path;
 pub mod revset;
 mod revset_parser;
 pub mod rewrite;
-#[cfg(feature = "testing")]
 pub mod secret_backend;
 pub mod settings;
 pub mod signing;
@@ -86,6 +85,8 @@ pub mod stacked_table;
 pub mod store;
 pub mod str_util;
 pub mod submodule_store;
+#[cfg(feature = "testing")]
+pub mod test_signing_backend;
 pub mod time_util;
 pub mod transaction;
 pub mod tree;

lib/src/signing.rs

@@ -25,6 +25,7 @@ use crate::backend::CommitId;
 use crate::gpg_signing::GpgBackend;
 use crate::settings::UserSettings;
 use crate::ssh_signing::SshBackend;
+use crate::test_signing_backend::TestSigningBackend;
 
 /// A status of the signature, part of the [Verification] type.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -167,6 +168,10 @@ impl Signer {
             // Box::new(X509Backend::from_settings(settings)?) as Box<dyn SigningBackend>,
         ];
 
+        if cfg!(feature = "testing") {
+            backends.push(Box::new(TestSigningBackend) as Box<dyn SigningBackend>);
+        }
+
         let main_backend = settings
             .signing_backend()
             .map(|backend| {

lib/testutils/src/test_signing_backend.rs → lib/src/test_signing_backend.rs

@@ -1,3 +1,20 @@
+// Copyright 2023 The Jujutsu Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Generic APIs to work with cryptographic signatures created and verified by
+//! various backends.
+
 use hex::ToHex;
 use jj_lib::content_hash::blake2b_hash;
 use jj_lib::signing::SigStatus;
@@ -6,6 +23,7 @@ use jj_lib::signing::SignResult;
 use jj_lib::signing::SigningBackend;
 use jj_lib::signing::Verification;
 
+/// A test signing backend that uses a simple hash-based signature format.
 #[derive(Debug)]
 pub struct TestSigningBackend;
 

lib/tests/test_signing.rs

@@ -7,9 +7,9 @@ use jj_lib::signing::SigStatus;
 use jj_lib::signing::SignBehavior;
 use jj_lib::signing::Signer;
 use jj_lib::signing::Verification;
+use jj_lib::test_signing_backend::TestSigningBackend;
 use test_case::test_case;
 use testutils::create_random_commit;
-use testutils::test_signing_backend::TestSigningBackend;
 use testutils::write_random_commit;
 use testutils::TestRepoBackend;
 use testutils::TestWorkspace;

lib/testutils/src/lib.rs

@@ -64,7 +64,6 @@ use tempfile::TempDir;
 use crate::test_backend::TestBackendFactory;
 
 pub mod test_backend;
-pub mod test_signing_backend;
 
 pub fn hermetic_libgit2() {
     // libgit2 respects init.defaultBranch (and possibly other config