Fix a few diagnostics
When working on the inline diagnostics conversion (https://github.com/rust-lang/rust/issues/151366), I noticed that my script sometimes took the wrong message.
Because it didn't happen very often, I just fixed it manually when a uitest fails.
However I got paranoid that the script changed messages that were not covered by uitests, so I checked for all messages in the previous `messages.ftl` files, whether they occured at least once in the codebase. I found 3 messages that indeed were wrongly replaced by my script, fixed them, and added uitests to make sure this doesn't happen again :)
r? @jdonszelmann (Anyone else, also feel free to review, just assigning to Jana because she's been reviewing the other PRs)
0d9b0ab620