have a test ensure that a panic-during-cleanup only prints one btacktrace by default

Author: RalfJung

tests/ui/backtrace.rs

@@ -106,6 +106,8 @@ fn runtest(me: &str) {
                 contains_verbose_expected(s, "double"),
             "bad output3: {}", s
         );
+        // Make sure it's only one stack trace.
+        assert_eq!(s.split("stack backtrace").count(), 2);
 
         // Make sure a stack trace isn't printed too many times
         //

tests/ui/panics/panic-in-cleanup.run.stderr

@@ -6,5 +6,4 @@ BOOM
 stack backtrace:
 thread 'main' panicked at library/core/src/panicking.rs:126:5:
 panic in a destructor during cleanup
-stack backtrace:
 thread caused non-unwinding panic. aborting.