fix: listbox tests no longer spew warnings

[scomea]

Description

Listbox tests were spewing warnings about attaching elements to document body. Created a container and used that instead.

Motivation & context

Warnings are bad...

Issue type checklist

• Chore: A change that does not impact distributed packages.
• Bug fix: A change that fixes an issue, link to the issue above.
• New feature: A change that adds functionality.

Is this a breaking change?

• This change causes current functionality to break.

Process & policy checklist

• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.

[codeclimate]

Code Climate has analyzed commit d67fad1 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 88.4% (0.0% change).

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