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/multi_buffer/Cargo.toml

@@ -27,7 +27,6 @@ clock.workspace = true
 collections.workspace = true
 ctor.workspace = true
 buffer_diff.workspace = true
-env_logger.workspace = true
 gpui.workspace = true
 itertools.workspace = true
 language.workspace = true
@@ -57,3 +56,4 @@ rand.workspace = true
 settings = { workspace = true, features = ["test-support"] }
 text = { workspace = true, features = ["test-support"] }
 util = { workspace = true, features = ["test-support"] }
+zlog.workspace = true

crates/multi_buffer/src/multi_buffer_tests.rs

@@ -11,9 +11,7 @@ use util::test::sample_text;
 
 #[ctor::ctor]
 fn init_logger() {
-    if std::env::var("RUST_LOG").is_ok() {
-        env_logger::init();
-    }
+    zlog::init_test();
 }
 
 #[gpui::test]