Fix bugs with multicursor completions (#28586)

Release Notes:

- Fixed completions with multiple cursors leaving duplicated prefixes.
- Fixed crash when accepting a completion in a multibuffer with multiple
cursors.
- Vim: improved `single-repeat` after accepting a completion, now
pressing `.` to replay the completion will re-insert the completion text
at the cursor position.

crates/editor/src/test.rs

@@ -1,8 +1,7 @@
 pub mod editor_lsp_test_context;
 pub mod editor_test_context;
 
-use std::sync::LazyLock;
-
+pub use crate::rust_analyzer_ext::expand_macro_recursively;
 use crate::{
     DisplayPoint, Editor, EditorMode, FoldPlaceholder, MultiBuffer,
     display_map::{DisplayMap, DisplaySnapshot, ToDisplayPoint},
@@ -11,11 +10,11 @@ use gpui::{
     AppContext as _, Context, Entity, Font, FontFeatures, FontStyle, FontWeight, Pixels, Window,
     font,
 };
+use pretty_assertions::assert_eq;
 use project::Project;
+use std::sync::LazyLock;
 use util::test::{marked_text_offsets, marked_text_ranges};
 
-pub use crate::rust_analyzer_ext::expand_macro_recursively;
-
 #[cfg(test)]
 #[ctor::ctor]
 fn init_logger() {
@@ -96,8 +95,12 @@ pub fn assert_text_with_selections(
     cx: &mut Context<Editor>,
 ) {
     let (unmarked_text, text_ranges) = marked_text_ranges(marked_text, true);
-    assert_eq!(editor.text(cx), unmarked_text);
-    assert_eq!(editor.selections.ranges(cx), text_ranges);
+    assert_eq!(editor.text(cx), unmarked_text, "text doesn't match");
+    assert_eq!(
+        editor.selections.ranges(cx),
+        text_ranges,
+        "selections don't match",
+    );
 }
 
 // RA thinks this is dead code even though it is used in a whole lot of tests