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.
`@rustbot` label T-libs A-filesystem A-testsuite O-windows-7 O-windows-msvc
d9ed86729b