Test repl completions

[Ericson2314]

#10778 reminded me that we don't test repl completions (which can be different from CLI completions, yay).

Doing so is not easy, because readline calls the completions callback, and doesn't want to do so when standard input is not a pseudo terminal. We'll have to either force readline to do it anyways (during testing), or create a pseudo terminal as part of the test.

[Mic92]

Some expect like test? What languages for testing do we have at our disposal?

[Ericson2314]

Right now the "functional" tests are written in bash, and I guess making some pseudo terminals in bash is a bit of a pain per https://unix.stackexchange.com/questions/179030/how-do-i-come-by-this-pty-and-what-can-i-do-with-it

[Mic92]

If we don't want to bloat the dev dependencies by adding TCL or python, using tmux scripting API might be an option.

[roberth]

I've written a separate check with expect; see #10799

If we don't want to bloat the dev dependencies by adding TCL or python, using tmux scripting API might be an option.

I wrote it before this issue was opened, but didn't manage to make a PR yet.
I don't think it's too bad as long as it's not in the "critical" dependencies, such as the dev shell, let alone package dependencies.