chore: prevent debugger tests from speaking to a backend (#4313)

# Description

## Problem\*

Resolves <!-- Link to GitHub Issue -->

## Summary\*

We can avoid speaking to a backend in the noir debugger tests as we can
just pass in an explicit expression width. (although as we're generating
brillig then expression width doesn't really matter)

## Additional Context



## Documentation\*

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[Exceptional Case]** Documentation to be submitted in a separate
PR.

# PR Checklist\*

- [x] I have tested the changes locally.
- [x] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

tooling/debugger/Cargo.toml

@@ -27,5 +27,4 @@ serde_json.workspace = true
 [dev-dependencies]
 assert_cmd = "2.0.12"
 rexpect = "0.5.0"
-test-binary = "3.0.1"
 tempfile.workspace = true

tooling/debugger/tests/debug.rs

@@ -5,29 +5,27 @@ mod tests {
 
     use rexpect::spawn_bash;
 
-    test_binary::build_test_binary_once!(mock_backend, "../backend_interface/test-binaries");
-
     // include tests generated by `build.rs`
     include!(concat!(env!("OUT_DIR"), "/debug.rs"));
 
     pub fn debugger_execution_success(test_program_dir: &str) {
         let nargo_bin =
             cargo_bin("nargo").into_os_string().into_string().expect("Cannot parse nargo path");
 
-        let mock_backend_path =
-            path_to_mock_backend().into_string().expect("Cannot parse mock_backend path");
-
         let mut dbg_session = spawn_bash(Some(10000)).expect("Could not start bash session");
 
+        // Set backend to `/dev/null` to force an error if nargo tries to speak to a backend.
         dbg_session
-            .send_line(&format!("export NARGO_BACKEND_PATH={}", mock_backend_path))
+            .send_line("export NARGO_BACKEND_PATH=/dev/null")
             .expect("Could not export NARGO_BACKEND_PATH.");
         dbg_session.wait_for_prompt().expect("Could not export NARGO_BACKEND_PATH.");
 
         // Start debugger and test that it loads for the given program.
         dbg_session
             .execute(
-                &format!("{} debug --program-dir {} --force-brillig", nargo_bin, test_program_dir),
+                &format!(
+                    "{nargo_bin} debug --program-dir {test_program_dir} --force-brillig --expression-width 3"
+                ),
                 ".*\\Starting debugger.*",
             )
             .expect("Could not start debugger");