When CTRL-C/keyboard-interrupt a hanging test pytest does not show a stacktrace. You can enable it via "--fulltrace" but that's not obvious. So we should add a hint when ctrl-c happens.

we could also show the traceback by default but sometimes one hits ctrl-c because one realizes the invocation was wrong or something and then showing long tracebacks by default is obnoxious. So i suggest we stick with how things are and add something to the message