add some completion tests (#12908)

# Description
```nushell
❯ ls
╭───┬───────┬──────┬──────┬──────────╮
│ # │ name  │ type │ size │ modified │
├───┼───────┼──────┼──────┼──────────┤
│ 0 │ a.txt │ file │  0 B │ now      │
╰───┴───────┴──────┴──────┴──────────╯

❯ ls a.
NO RECORDS FOUND
```

There is a completion issue on previous version, I think @amtoine have
reproduced it before. But currently I can't reproduce it on latest main.
To avoid such regression, I added some tests for completion.

---------

Co-authored-by: Antoine Stevan <44101798+amtoine@users.noreply.github.com>

crates/nu-cli/tests/completions/mod.rs

@@ -292,6 +292,8 @@ fn partial_completions() {
 
     // Create the expected values
     let expected_paths: Vec<String> = vec![
+        file(dir.join("partial_a").join("have_ext.exe")),
+        file(dir.join("partial_a").join("have_ext.txt")),
         file(dir.join("partial_a").join("hello")),
         file(dir.join("partial_a").join("hola")),
         file(dir.join("partial_b").join("hello_b")),
@@ -310,6 +312,8 @@ fn partial_completions() {
     // Create the expected values
     let expected_paths: Vec<String> = vec![
         file(dir.join("partial_a").join("anotherfile")),
+        file(dir.join("partial_a").join("have_ext.exe")),
+        file(dir.join("partial_a").join("have_ext.txt")),
         file(dir.join("partial_a").join("hello")),
         file(dir.join("partial_a").join("hola")),
         file(dir.join("partial_b").join("hello_b")),
@@ -360,6 +364,34 @@ fn partial_completions() {
 
     // Match the results
     match_suggestions(expected_paths, suggestions);
+
+    // Test completion for all files under directories whose names begin with "pa"
+    let file_str = file(dir.join("partial_a").join("have"));
+    let target_file = format!("rm {file_str}");
+    let suggestions = completer.complete(&target_file, target_file.len());
+
+    // Create the expected values
+    let expected_paths: Vec<String> = vec![
+        file(dir.join("partial_a").join("have_ext.exe")),
+        file(dir.join("partial_a").join("have_ext.txt")),
+    ];
+
+    // Match the results
+    match_suggestions(expected_paths, suggestions);
+
+    // Test completion for all files under directories whose names begin with "pa"
+    let file_str = file(dir.join("partial_a").join("have_ext."));
+    let file_dir = format!("rm {file_str}");
+    let suggestions = completer.complete(&file_dir, file_dir.len());
+
+    // Create the expected values
+    let expected_paths: Vec<String> = vec![
+        file(dir.join("partial_a").join("have_ext.exe")),
+        file(dir.join("partial_a").join("have_ext.txt")),
+    ];
+
+    // Match the results
+    match_suggestions(expected_paths, suggestions);
 }
 
 #[test]
@@ -394,6 +426,13 @@ fn command_ls_with_filecompletion() {
         ".hidden_folder/".to_string(),
     ];
 
+    match_suggestions(expected_paths, suggestions);
+
+    let target_dir = "ls custom_completion.";
+    let suggestions = completer.complete(target_dir, target_dir.len());
+
+    let expected_paths: Vec<String> = vec!["custom_completion.nu".to_string()];
+
     match_suggestions(expected_paths, suggestions)
 }
 #[test]
@@ -428,6 +467,13 @@ fn command_open_with_filecompletion() {
         ".hidden_folder/".to_string(),
     ];
 
+    match_suggestions(expected_paths, suggestions);
+
+    let target_dir = "open custom_completion.";
+    let suggestions = completer.complete(target_dir, target_dir.len());
+
+    let expected_paths: Vec<String> = vec!["custom_completion.nu".to_string()];
+
     match_suggestions(expected_paths, suggestions)
 }