[MINOR] Resolve "Windows SafeContract Deletion panic"

MERGE REQUEST

MR Guidelines

Overview

This MR fixes contractset related tests failing on windows due to file handles not being closed correctly.

A failing test was run repeatedly on the windows runner to verify the issue is resolved.

https://gitlab.com/NebulousLabs/Sia/-/jobs/955284267

Checklist

Review and complete the checklist to ensure that the MR is complete before assigned to an approver.

• All new methods or updated methods have clear docstrings
• Testing added or updated for new methods
• Any new packages are added to Makefile and .gitlab-ci.yml
• API documentation updated for API updates
• Module README.md updated for changes to workflow
• Issue added to Sia-UI repo for new supporting features
• Changelog File Created

Issues Closed

Closes #4473 (closed)