Make htex scale_in pick longest idle blocks (#3135)

Prior to this, scale_in picked the shortest-idle (i.e. most recently used but
now idle) blocks.

In the scaling code right now, there isn't any particular reason to order
idle blocks in any particular order...

... but the next PR following on from this one will also consider unstarted
blocks with infinite idle time, and in that situation, the policy will be that
unstarted idle blocks should be scaled down in preference to currently
running blocks. (other policies are possible and reasonable but not addressed
by either this PR or the upcoming PR)

I think this was intended to work this way to begin with (although I'm unsure
of the justification) but there is a sign error that is flipped by this PR.

parsl/executors/high_throughput/executor.py

@@ -739,7 +739,12 @@ class BlockInfo:
             block_info[b_id].tasks += manager['tasks']
             block_info[b_id].idle = min(block_info[b_id].idle, manager['idle_duration'])
 
-        sorted_blocks = sorted(block_info.items(), key=lambda item: (item[1].idle, item[1].tasks))
+        # The scaling policy is that longest idle blocks should be scaled down
+        # in preference to least idle (most recently used) blocks.
+        # Other policies could be implemented here.
+
+        sorted_blocks = sorted(block_info.items(), key=lambda item: (-item[1].idle, item[1].tasks))
+
         logger.debug(f"Scale in selecting from {len(sorted_blocks)} blocks")
         if max_idletime is None:
             block_ids_to_kill = [x[0] for x in sorted_blocks[:blocks]]