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/assistant_slash_commands/Cargo.toml

@@ -44,6 +44,6 @@ worktree.workspace = true
 workspace-hack.workspace = true
 
 [dev-dependencies]
-env_logger.workspace = true
 pretty_assertions.workspace = true
 settings.workspace = true
+zlog.workspace = true

crates/assistant_slash_commands/src/file_command.rs

@@ -587,9 +587,7 @@ mod test {
     use super::collect_files;
 
     pub fn init_test(cx: &mut gpui::TestAppContext) {
-        if std::env::var("RUST_LOG").is_ok() {
-            env_logger::try_init().ok();
-        }
+        zlog::init_test();
 
         cx.update(|cx| {
             let settings_store = SettingsStore::test(cx);