Fix: Memory leak in WindowDescTestsFixture

[JGRennison]

Motivation / Problem

Fix memory leak in WindowDescTestsFixture so that the various tests can be run with memory checkers enabled, without unnecessarily failing the tests.

Description

Delete widget tree returned from MakeWindowNWidgetTree.

Limitations

Doesn't fix errors/warnings for other sanitiser types (e.g. undefined behaviour).

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker