rust: Move open_file test to utils module

We had essentially the same test in the `lockfile` and `treefile`
modules. Just dedupe those and move it to `utils`.

Closes: #1867
Approved by: cgwalters

rust/src/lockfile.rs

@@ -180,17 +180,6 @@ mod tests {
             Ok(_) => panic!("Expected invalid lockfile"),
         }
     }
-
-    #[test]
-    fn test_open_file_nonexistent() {
-        let path = "/usr/share/empty/manifest.json";
-        match lockfile_parse(path) {
-            Err(ref e) => assert!(e
-                .to_string()
-                .starts_with(format!("Can't open file {:?}:", path).as_str())),
-            Ok(_) => panic!("Expected nonexistent lockfile error for {}", path),
-        }
-    }
 }
 
 mod ffi {

rust/src/treefile.rs

@@ -1125,17 +1125,6 @@ add-commit-metadata:
         assert!(data.get("my-third-key").unwrap().as_i64().unwrap() == 1000);
         assert!(data.get("my-fourth-key").unwrap().as_object().unwrap().get("nested").unwrap().as_str().unwrap() == "table");
     }
-
-    #[test]
-    fn test_open_file_nonexistent() {
-        let path = "/usr/share/empty/manifest.yaml";
-        match treefile_parse(path, None) {
-            Err(ref e) => assert!(e
-                .to_string()
-                .starts_with(format!("Can't open file {:?}:", path).as_str())),
-            Ok(_) => panic!("Expected nonexistent treefile error for {}", path),
-        }
-    }
 }
 
 mod ffi {

rust/src/utils.rs

@@ -146,6 +146,17 @@ mod tests {
         let r = varsubst("${osvendor}", &subs).unwrap();
         assert_eq!(r, "fedora");
     }
+
+    #[test]
+    fn test_open_file_nonexistent() {
+        let path = "/usr/share/empty/manifest.yaml";
+        match open_file(path) {
+            Err(ref e) => assert!(e
+                .to_string()
+                .starts_with(format!("Can't open file {:?} for reading:", path).as_str())),
+            Ok(_) => panic!("Expected nonexistent treefile error for {}", path),
+        }
+    }
 }
 
 mod ffi {