Move Network test helper from util crate into text crate

This way, `util` does not depend on `clock`.

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

crates/text/src/text.rs

@@ -1,5 +1,7 @@
 mod anchor;
 pub mod locator;
+#[cfg(any(test, feature = "test-support"))]
+pub mod network;
 pub mod operation_queue;
 mod patch;
 mod point;