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
