Implement an internal "assertShort" utility (#16164)
Implement an internal "assertShort" utility (#16164)

I'm often implementing against an assertion, for instance in
TreeChecker. While implementing a fix I add some extra output. So it's
useful for me to suppress the full page of stacktrace that that assert
typically emits, so I can roundtrip without having to scroll back up.
sjrd/dotty,lampepfl/dotty,lampepfl/dotty,dotty-staging/dotty,sjrd/dotty,sjrd/dotty,dotty-staging/dotty,lampepfl/dotty,lampepfl/dotty,dotty-staging/dotty,sjrd/dotty,sjrd/dotty,dotty-staging/dotty,dotty-staging/dotty,lampepfl/dotty