tests: double check if tests contain incorrect follow-up error annotation syntax `//|` instead of `//~|`

[jieyouxu]

If e.g. //| HELP is used, I believe it is just disregarded by compiletest and no help message is actually checked.

[jieyouxu]

Actually a cursory search shows just tests/ui/return/tail-expr-as-potential-return.rs being the one test that seems to have this problem.