Fix and test for RT #123520.

S17-promise/in.t

@@ -1,7 +1,21 @@
 use v6;
 use Test;
 
-plan 2;
+plan 3;
+
+# RT #123520
+# This test comes first in the file so it's the first to start any threads;
+# this means we can test the actual bug in the ticket.
+{
+    my @order;
+    await Promise.anyof(
+        start({ sleep 3; @order.push(3) }),
+        Promise.in(1).then({ @order.push(1) }));
+    sleep 1;
+    @order.push(2);
+    sleep 2;
+    is @order, (1, 2, 3), 'Scheduler did not cause things to run in wrong order';
+}
 
 {
     my $start = now;