test: fvi pipeline tests hold the Drive-holder lock; the lock exists under loom too

src/halt.rs

@@ -538,7 +538,8 @@ pub fn live_drive_holders() -> usize {
 }
 
 // Every test that spawns a Drive holder or reads the process-wide count holds this.
-#[cfg(all(test, not(loom)))]
+#[cfg(test)]
+#[cfg_attr(loom, allow(dead_code))]
 pub(crate) static DRIVE_HOLDER_TEST_LOCK: std::sync::Mutex<()> = std::sync::Mutex::new(());
 
 /// A thread that holds the Drive (§2.5): the op joins it before returning. In debug

src/mux/fvi_pipeline_tests.rs

@@ -171,6 +171,7 @@ fn two_gop_image() -> Vec<u8> {
 
 /// Drive the real highway and write every frame into the `fvi://` sink.
 fn run_to_fvi(image: Vec<u8>, title: DiscTitle, path: &std::path::Path) {
+    let _drive = crate::sector::prefetched::holder_test_lock();
     let mut input = build_iso_pipeline(
         MemSource { data: image },
         title.clone(),