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/semantic_index/examples/index.rs

@@ -12,7 +12,7 @@ use std::{
 };
 
 fn main() {
-    env_logger::init();
+    zlog::init();
 
     use clock::FakeSystemClock;