Add new trap/errexit/signal test simulating interactive usage

Add new trap/errexit/signal test simulating interactive usage

First try add capturing use cases like the bin/ksh/main.c r1.52 one:
	"[...] we would not run the signal trap if,
	for example, ^C was pressed and -e was set."

With so many different factors (errexit?, receiving (special) signals,
traps themselves failing/returing non-zero, running child processes?, etc.)
it is very easy to miss subtle behaviour changes during scripting...