fmt: fix clippy warnings on all targets

Drop an unused test fixture constant, move download_trust_ppa_key above
the test module that precedes it, and simplify two test borrows.

src/apt/release.rs

@@ -1080,8 +1080,6 @@ EwQbAcFaKe4rlLx9e/EXAMPLEEXAMPLEEXAMPLEEXAMPLEEXAMPLEEXAMPLEEXAMPLE
 
     /// sha256(b"test")
     const SHA256_TEST: &str = "9f86d081884c7d659a2feaa0c55ad015a3bf4f1b2b0b822cd15d6c15b0f00a08";
-    /// sha256(b"")
-    const SHA256_EMPTY: &str = "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855";
 
     fn verified_noble() -> VerifiedRelease {
         let body = split_clearsigned(NOBLE_INRELEASE).unwrap();
@@ -1241,9 +1239,9 @@ garbage
         // reported as GpgvMissing, distinct from a signature failure
         let status = run_gpgv(
             "/nonexistent/pkh-test-gpgv",
-            &Path::new("/unused/scratch").to_path_buf(),
+            Path::new("/unused/scratch"),
             &[PathBuf::from("/unused/keyring.gpg")],
-            &Path::new("/unused/InRelease"),
+            Path::new("/unused/InRelease"),
             None,
         );
         assert!(matches!(status, GpgvStatus::GpgvMissing(_)));