Add embedded editor tests

[TiBiBa]

Start the name of the PR with one of the relevant prefixed in the title of the PR:

• [LANGUAGE] -> changes related to grammars or the transpiler
• [FIX] -> solving a bug
• [CHORE] -> things unrelated to the code, such as mergify scripts
• [REFACTORING] -> should not change anything, cleanup of code
• [UI] -> improvements and changes of the Hedy user interface
• [TESTING] -> changes made to the test suite
• [BACKEND] -> changes to the internal logic of the Hedy website

Finish the name of the PR with a clear description of the changes and delete the lines above from the description.

Now fill out the remainder of this template by replacing all italic content

Description

Changes in detail. For example: "Adds translations of levels 1 to 12 to Polish" (note the use of present tense!)

Fixes issue or discussion number

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Tip, if you use the word fixes before the issue number in this description, the related issue will automatically close then the PR is merged!

How to test

• If this is a UI change: describe how to run your code to see it in action. See this Stop long running programs in client-side JS #880 (comment) for an example
• If this is a language change: add a few tests showing the difference

Checklist
Done? Check if you have it all in place using this list: (mark with x if done)

• Contains one of the PR categories in the name
• Describes changes in the format above
• Links to an existing issue or discussion
• Has a "How to test" section

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[Felienne]

Hi @TiBiBa!

We are trying to clean up our PRs a bit, do you plan to move this forward? I see a lot of value for you in adding tests so that we don't break the LearnBeat integration, but we are not going to pick this up ourselves I think.