Auto merge of #1184 - divergentdave:rename-error-test, r=RalfJung

Test error case of std::fs::rename

As suggested [here](https://github.com/rust-lang/miri/pull/1158#issuecomment-586459463) this PR adds an additional test case for calling rename on a file path that doesn't exist.

tests/run-pass/fs.rs

@@ -169,8 +169,16 @@ fn test_rename() {
 
     let file = File::create(&path1).unwrap();
     drop(file);
+
+    // Renaming should succeed
     rename(&path1, &path2).unwrap();
+    // Check that the old file path isn't present
     assert_eq!(ErrorKind::NotFound, path1.metadata().unwrap_err().kind());
+    // Check that the file has moved successfully
     assert!(path2.metadata().unwrap().is_file());
+
+    // Renaming a nonexistent file should fail
+    assert_eq!(ErrorKind::NotFound, rename(&path1, &path2).unwrap_err().kind());
+
     remove_file(&path2).unwrap();
 }