discrepancy after switching to the new REPL internals was
discovered also on IJ plugin tests, so added a test emulating it.
also some test infra refactoring
- render the whole line where the error/warning points to, if any, and another
line with '^', like other compilers do
- lowercase diagnostic severity
- decapitalize the message if it doesn't start with a proper name
Ilya Chernikov