test for forgetting locked mutex

tests/pass/concurrency/mutex_leak.rs

@@ -0,0 +1,9 @@
+// compile-flags: -Zmiri-ignore-leaks
+use std::mem;
+use std::sync::Mutex;
+
+fn main() {
+    // Test for https://github.com/rust-lang/rust/issues/85434
+    let m = Mutex::new(5i32);
+    mem::forget(m.lock());
+}