(PUP-12061) Add test cases for runinterval and splaylimit interaction

Since splaylimit defaults to runinterval, changes to runinterval affect splay.
Add tests covering cases where runinterval is increased and decreased. It's
important to recalculate splay to reduce thundering herds. For example, if
runinterval is changed from 30 mins to 60 mins, we want the compute a new slot
in the 60 min interval to decrease server load.

lib/puppet/daemon.rb

@@ -173,6 +173,8 @@ def run_event_loop
     reparse_run = Puppet::Scheduler.create_job(Puppet[:filetimeout]) do
       Puppet.settings.reparse_config_files
       agent_run.run_interval = Puppet[:runinterval]
+      # Puppet[:splaylimit] defaults to Puppet[:runinterval] so if runinterval
+      # changes, but splaylimit doesn't, we'll still recalculate splay
       agent_run.splay_limit = Puppet[:splay] ? Puppet[:splaylimit] : 0
       if Puppet[:filetimeout] == 0
         reparse_run.disable

spec/unit/daemon_spec.rb

@@ -135,6 +135,30 @@ def run_loop(jobs)
 
         expect(agent_run.splay).to eq(0)
       end
+
+      it "recalculates splay when runinterval is decreased" do
+        Puppet[:runinterval] = 60
+        daemon.start
+
+        Puppet[:runinterval] = Puppet[:runinterval] - 30
+        new_splay = agent_run.splay + 1
+        allow(agent_run).to receive(:rand).and_return(new_splay)
+        reparse_run.run(Time.now)
+
+        expect(agent_run.splay).to eq(new_splay)
+      end
+
+      it "recalculates splay when runinterval is increased" do
+        Puppet[:runinterval] = 60
+        daemon.start
+
+        Puppet[:runinterval] = Puppet[:runinterval] + 30
+        new_splay = agent_run.splay - 1
+        allow(agent_run).to receive(:rand).and_return(new_splay)
+        reparse_run.run(Time.now)
+
+        expect(agent_run.splay).to eq(new_splay)
+      end
     end
   end