Add test case for issue#6099

[shulaoda]

closes #6099

This one is a little different because const BAR = 42; will be rejected by the parser. You should see a similar error message if you run rustfmt on that input:

error: missing type for `const` item

 --> <stdin>:1:10

  |

1 | const BAR = 42;

  |          ^ help: provide a type for the item: `: <type>`

You should be able to add another parser error test to src/test/parser.rs

Originally posted by @ytmimi in #6099

[jieyouxu]

Same as #6692 (comment), there's already a regression test for the root cause of not explicitly handling stashed diagnostics. I don't think these explicit cases meaningfully improve the coverage, because, well, it's the same root cause.

Regardless, thanks for the PR!