Fix compile on Windows

Utf8Error is Unix-only. For now, there are no tests that actually
trigger this LOC anyways, so just make it panic.

The problem is our trait returns a ReadlineError which is used by the
shell to do things like interpret ^C and ^D. So we can't do something
like 1/ use `anyhow::Result` or 2/ make Err an associated type on Ui,
because the shell needs to know *something* about the type of Error.

This can be fixed, but we'll need another layer of abstraction. For now,
I just wanted to fix the build.

src/ui.rs

@@ -57,7 +57,7 @@ pub mod testing {
         fn user_input(&self) -> Result<String, ReadlineError> {
             let mut inner = self.inner.borrow_mut();
             if inner.pending.is_empty() {
-                return Err(ReadlineError::Utf8Error);
+                panic!("mock is not ready for user input");
             }
             let remaining = inner.pending.split_off(1);
             let first = inner.pending.pop().unwrap();