Add a test to cover failure + callsame issue.

See comment in the test for details.

integration/failure-and-callsame.t

@@ -0,0 +1,35 @@
+use Test;
+
+# This test covers a nasty once-bug involving callsame and Failure, where the
+# finalization of Failure could steal the target of a callsame and so lead to
+# delegation not taking place. Golfed from a bug report on OO::Monitors, which
+# turned out to be a bug in Rakudo. This at the time of adding it would fail
+# even without any GC stress applied provided a decent number of iterations
+# are done (thus the 50 repetitions; it relies on unfortunate timing).
+
+plan 50;
+
+class Grid {
+    has @.grid;
+
+    method change-cell($x, $s) {
+        @!grid[$x] = $s;
+    }
+}
+
+my $leaving = 0;
+Grid.^lookup('change-cell').wrap(-> \SELF, | {
+    LEAVE ++$leaving;
+    callsame
+});
+
+my $grid = Grid.new();
+for ^50 {
+    $grid.grid = 'BUG' xx 100;
+    for ^(100 div 2) -> $x {
+        Failure.new() // ''; # Triggered glitches
+        $grid.change-cell($x * 2,     'a');
+        $grid.change-cell($x * 2 + 1, 'b');
+    }
+    ok $grid.grid.join !~~ /BUG/, 'callsame + Failure glitch not observed';
+}