transpile: set -o explicitly for rustc in snapshot tests

Since `-o /dev/null` doesn't work since intermediates are created in the same directly,
we compile to the normal `.rlib` output path and then delete it.
This just makes that explicit with `-o` so it's more obviously correct,
and we can name the `.rlib` anything we want as well.

Author: kkysen

c2rust-transpile/tests/snapshots.rs

@@ -69,18 +69,18 @@ fn transpile(platform: Option<&str>, c_path: &Path) {
 
     insta::assert_snapshot!(name, &rs, &debug_expr);
 
-    let status = Command::new("rustc")
-        .args(&["--crate-type", "lib", "--edition", "2021"])
-        .arg(&rs_path)
-        .status();
-    assert!(status.unwrap().success());
     let rlib_path = {
         let mut file_name = OsString::new();
         file_name.push("lib");
         file_name.push(rs_path.file_stem().unwrap());
         file_name.push(".rlib");
         PathBuf::from(file_name)
     };
+    let status = Command::new("rustc")
+        .args(&["--crate-type", "lib", "--edition", "2021", "-o"])
+        .args(&[&rlib_path, &rs_path])
+        .status();
+    assert!(status.unwrap().success());
     fs::remove_file(&rlib_path).unwrap();
 }