test: Check if a translation macro actually exists

[mal-tee]

We should also write a test that checks if a macro is defined, if it's used in the translations. Feel free to move this into a new issue.

Originally posted by @mal-tee in #802 (comment)

[baltpeter]

Can you elaborate on what you mean by that?

[mal-tee]

If a translation string uses ${x}, x should be defined in the macro section. :D

[baltpeter]

But that's the other way compared to what the issue says. :P

Either way, how would you implement that? I don't see a way to do that through Cypress (other than what I already did in #802). Do you want to establish another testing framework?