Auto merge of #6787 - matthiaskrgr:lint_msgs, r=llogiq

Auto merge of #6787 - matthiaskrgr:lint_msgs, r=llogiq

tests: add test that roughly ensures that our lint messages conform with the diagnostics convention of the rustc dev guide

lint message should not start with uppercase letters
lint messages should not have punctuation at the end of the last line

https://rustc-dev-guide.rust-lang.org/diagnostics.html#diagnostic-structure

The test reads through all the .stderr files in the testsuit and checks lint messages that start with "help: ", "error: " etc.
There is also an exception list for special messages that are deemed acceptable.

changelog: make sure lint messages conform with the rustc dev guide and add test