test: Fix debugger repl tests

This makes the output of simple/test-debugger-repl and
simle/test-debugger-repl-utf8 mirror an actual debugger session, so it's
a bit easier to reason about.

Also, it uses the same code for both, and fixes it so that it doesn't
leave zombie processes lying around when it crashes.

Run 1000 times without any failures or zombies.