Move worktree tests into their own file (#2608)

The worktree has a lot of tests. Let's reduce their indentation, and
make it easier to navigate to them. We deserve it.

Just doing this in its own PR, to minimize conflicts.

Release Notes:

- N/A

crates/project/src/project.rs

@@ -7,6 +7,8 @@ pub mod worktree;
 
 #[cfg(test)]
 mod project_tests;
+#[cfg(test)]
+mod worktree_tests;
 
 use anyhow::{anyhow, Context, Result};
 use client::{proto, Client, TypedEnvelope, UserStore};