Add test that exercises ReentrantMutex

tests/run-pass/reentrant-println.rs

@@ -0,0 +1,17 @@
+use std::fmt::{Display, Error, Formatter};
+
+// This test case exercises std::sys_common::remutex::ReentrantMutex
+// by calling println!() from inside fmt
+
+struct InterruptingCow();
+
+impl Display for InterruptingCow {
+    fn fmt(&self, _f: &mut Formatter<'_>) -> Result<(), Error> {
+        println!("Moo");
+        Ok(())
+    }
+}
+
+fn main() {
+    println!("\"Knock knock\" \"Who's {} there?\"", InterruptingCow());
+}

tests/run-pass/reentrant-println.stdout

@@ -0,0 +1,2 @@
+"Knock knock" "Who's Moo
+ there?"