Test(fs): Fix test_eq_windows_file_type for Windows 7

Would otherwise fail on:

```
thread 'fs::tests::test_eq_windows_file_type' panicked at library/std/src/test_helpers.rs:53:20:
called `Result::unwrap()` on an `Err` value: Os { code: 5, kind: PermissionDenied, message: "Access is denied." }
```

This came from the read-only attribute set on the test file. In order to
fix this, instead of simply disabling the test, the attribute is reset
before the test's end so it may still run successfully.

Signed-off-by: Paul Mabileau <paul.mabileau@harfanglab.fr>

Author: PaulDance

library/std/src/fs/tests.rs

@@ -1782,8 +1782,30 @@ fn test_eq_windows_file_type() {
     // Change the readonly attribute of one file.
     let mut perms = file1.metadata().unwrap().permissions();
     perms.set_readonly(true);
-    file1.set_permissions(perms).unwrap();
+    file1.set_permissions(perms.clone()).unwrap();
+    #[cfg(target_vendor = "win7")]
+    let _g = ReadonlyGuard { file: &file1, perms };
     assert_eq!(file1.metadata().unwrap().file_type(), file2.metadata().unwrap().file_type());
+
+    // Reset the attribute before the `TmpDir`'s drop that removes the
+    // associated directory, which fails with a `PermissionDenied` error when
+    // running under Windows 7.
+    #[cfg(target_vendor = "win7")]
+    struct ReadonlyGuard<'f> {
+        file: &'f File,
+        perms: fs::Permissions,
+    }
+    #[cfg(target_vendor = "win7")]
+    impl<'f> Drop for ReadonlyGuard<'f> {
+        fn drop(&mut self) {
+            self.perms.set_readonly(false);
+            let res = self.file.set_permissions(self.perms.clone());
+
+            if !thread::panicking() {
+                res.unwrap();
+            }
+        }
+    }
 }
 
 /// Regression test for https://github.com/rust-lang/rust/issues/50619.