Update the test code to newer realities (#15084)

Follow-up of https://github.com/zed-industries/zed/pull/14886

We do not require the branch to be up-to-date with `main` before
merging, and in 4 days some related test code got reworked so that there
were no conflicts and it slipped.

Release Notes:

- N/A

crates/assistant/src/inline_assistant.rs

@@ -2789,8 +2789,9 @@ mod tests {
 
     #[gpui::test(iterations = 10)]
     async fn test_autoindent_respects_tabs_in_selection(cx: &mut TestAppContext) {
+        cx.update(LanguageModelRegistry::test);
+        cx.update(completion::LanguageModelCompletionProvider::test);
         cx.set_global(cx.update(SettingsStore::test));
-        cx.update(|cx| FakeCompletionProvider::setup_test(cx));
         cx.update(language_settings::init);
 
         let text = indoc! {"