Extract a TextBuffer from Buffer, which has no tree or file

Co-Authored-By: Antonio Scandurra <me@as-cii.com>
Co-Authored-By: Nathan Sobo <nathan@zed.dev>

crates/editor/src/display_map.rs

@@ -436,7 +436,7 @@ mod tests {
                     }
                 }
                 _ => {
-                    buffer.update(&mut cx, |buffer, cx| buffer.randomly_mutate(&mut rng, cx));
+                    buffer.update(&mut cx, |buffer, cx| buffer.randomly_edit(&mut rng, 5, cx));
                 }
             }