sign: fix signature in TestSignatureBackend::sign()

The following test was failing:
```
---- test_signature_templates::test_signature_templates stdout ----
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ Snapshot Summary ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Snapshot: signature_templates
Source: cli/tests/test_signature_templates.rs:28
────────────────────────────────────────────────────────────────────────────────
Expression: stdout
────────────────────────────────────────────────────────────────────────────────
-old snapshot
+new results
────────────┬───────────────────────────────────────────────────────────────────
    0     0 │ @  Commit ID: 05ac066d05701071af20e77506a0f2195194cbc9
    1     1 │ │  Change ID: qpvuntsmwlqtpsluzzsnyyzlmlwvmlnu
    2     2 │ │  Author: Test User <[email protected]> (2001-02-03 08:05:07)
    3     3 │ │  Committer: Test User <[email protected]> (2001-02-03 08:05:07)
    4       │-│  Signature: Good test signature
          4 │+│  Signature: Bad test signature
    5     5 │ │
    6     6 │ │      (no description set)
    7     7 │ │
    8     8 │ ◆  Commit ID: 0000000000000000000000000000000000000000
────────────┴───────────────────────────────────────────────────────────────────
```

Print debugging revealed that the signature was bad, because of a
missing trailing `\n` in `TestSignatureBackend::sign()`.

```diff
diff --git a/lib/src/test_signing_backend.rs b/lib/src/test_signing_backend.rs
index d47fef1086..0ba249e358 100644
--- a/lib/src/test_signing_backend.rs
+++ b/lib/src/test_signing_backend.rs
@@ -59,6 +59,8 @@
         let key = (!key.is_empty()).then_some(std::str::from_utf8(key).unwrap().to_owned());

         let sig = self.sign(data, key.as_deref())?;
+        dbg!(&std::str::from_utf8(&signature).unwrap());
+        dbg!(&std::str::from_utf8(&sig).unwrap());
         if sig == signature {
             Ok(Verification::new(
                 SigStatus::Good,
```

```
[lib/src/test_signing_backend.rs:62:9] &std::str::from_utf8(&signature).unwrap() = \"--- JJ-TEST-SIGNATURE ---\\nKEY: \\n5300977ff3ecda4555bd86d383b070afac7b7459c07f762af918943975394a8261d244629e430c8554258904f16dd9c18d737f8969f2e7d849246db0d93cc004\\n\"
[lib/src/test_signing_backend.rs:63:9] &std::str::from_utf8(&sig).unwrap() = \"--- JJ-TEST-SIGNATURE ---\\nKEY: \\n5300977ff3ecda4555bd86d383b070afac7b7459c07f762af918943975394a8261d244629e430c8554258904f16dd9c18d737f8969f2e7d849246db0d93cc004\"
```

lib/src/test_signing_backend.rs

@@ -46,7 +46,7 @@ impl SigningBackend for TestSigningBackend {
 
         let hash: String = blake2b_hash(&body).encode_hex();
 
-        Ok(format!("{PREFIX}{key}\n{hash}").into_bytes())
+        Ok(format!("{PREFIX}{key}\n{hash}\n").into_bytes())
     }
 
     fn verify(&self, data: &[u8], signature: &[u8]) -> SignResult<Verification> {