Why this change?
Currently, is it hard to iteratively write a theme settings migrations
because our theme migrations system does not rollback. Therefore, we
want to allow theme developers to be able to write QUnit tests for their
theme migrations files enabling them to iteratively write their theme
migrations.
What does this change do?
1. Update `Theme#baked_js_tests_with_digest` to include all `ThemeField`
records of `ThemeField#target` equal to `migrations`. Note that we do
not include the `settings` and `themePrefix` variables for migration files.
2. Don't minify JavaScript test files becasue it makes debugging in
development hard.
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