Register Markdown language in some tests to silence error logs (#13066)

This PR registers the Markdown language in some of the tests in the
`zed` crate to silence the error logs about the language not being found
when the chat panel attempts to load it.

Release Notes:

- N/A

crates/collab_ui/src/chat_panel/message_editor.rs

@@ -1,4 +1,4 @@
-use anyhow::Result;
+use anyhow::{Context, Result};
 use channel::{ChannelChat, ChannelStore, MessageParams};
 use client::{UserId, UserStore};
 use collections::HashSet;
@@ -133,7 +133,7 @@ impl MessageEditor {
 
         let markdown = language_registry.language_for_name("Markdown");
         cx.spawn(|_, mut cx| async move {
-            let markdown = markdown.await?;
+            let markdown = markdown.await.context("failed to load Markdown language")?;
             buffer.update(&mut cx, |buffer, cx| {
                 buffer.set_language(Some(markdown), cx)
             })