zlog: Replace usages of env_logger in tests with zlog (#31436)

Also fixes:
https://github.com/zed-industries/zed/pull/31400#issuecomment-2908165249

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/buffer_diff/src/buffer_diff.rs

@@ -1346,9 +1346,7 @@ mod tests {
 
     #[ctor::ctor]
     fn init_logger() {
-        if std::env::var("RUST_LOG").is_ok() {
-            env_logger::init();
-        }
+        zlog::init_test();
     }
 
     #[gpui::test]