fix(themes): fix incorrect audio container and add unit testing

[ReenigneArcher]

Description

This "should" fix themes being downloaded in the wrong audio container. Currently they are being downloaded in WebM, this changes that to be an Mp4 container with no video stream.

Additionally, unit testing is added using Xunit.

Screenshot

Issues Fixed or Closed

• Fixes All downloaded mp3 get corrupted #97

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Dependency update (updates to dependencies)
• Documentation update (changes to documentation)
• Repository update (changes to repository files, e.g. .github/...)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the in code docstring/documentation-blocks for new or existing methods/components

Branch Updates

LizardByte requires that branches be up-to-date before merging. This means that after any PR is merged, this branch
must be updated before it can be merged. You must also
Allow edits from maintainers.

• I want maintainers to keep my branch updated

[ReenigneArcher]

Todo:

• add dotnet test to CI
• add code coverage

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