Sanitize fields on the test data form

[slo248]

Description

Add validation on fields of input and output files if the problem's creator wants the reading and writing through files instead of the standard one.

Checklist

• [x] I have explained the purpose of this PR.
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the README/documentation
• [ ] Any dependent changes have been merged and published in downstream modules
• [ ] Informed of breaking changes, testing and migrations (if applicable).
• [x] Attached screenshots (if applicable).

By submitting this pull request, I confirm that my contribution is made under the terms of the AGPL-3.0 License.