Tests should be updated to be more idiomatic for vitest

[toasted-nutbread]

I've done some work on this in #461, #415, #363, etc. but a lot of the tests are not idiomatic for how vitest likes tests. I know this is because they were converted from the way I did tests in Yomichan, which was largely without a framework, but it would be useful to seem them get a full update to how vitest usually describes tests.

This would also help avoid issues where tests might silently not be working as expected such as #363, and could help encourage contributors to add more tests as there would be better code to reference.

Remaining files:

• test/cache-map.test.js
• test/core.test.js
• test/css-json.test.js
• test/hotkey-util.test.js
• test/japanese-util.test.js
• test/json-schema.test.js
• test/language-transformer.test.js
• test/object-property-accessor.test.js
• test/options-util.test.js
• test/profile-conditions-util.test.js
• test/text-source-map.test.js