Switch plugin API to use an accumulator

plugins/system/mock_PS.go

@@ -1,9 +1,9 @@
 package system
 
-import (
-	"github.com/influxdb/tivan/plugins/system/ps/load"
-	"github.com/stretchr/testify/mock"
-)
+import "github.com/stretchr/testify/mock"
+
+import "github.com/influxdb/tivan/plugins/system/ps/cpu"
+import "github.com/influxdb/tivan/plugins/system/ps/load"
 
 type MockPS struct {
 	mock.Mock
@@ -17,3 +17,11 @@ func (m *MockPS) LoadAvg() (*load.LoadAvgStat, error) {
 
 	return r0, r1
 }
+func (m *MockPS) CPUTimes() ([]cpu.CPUTimesStat, error) {
+	ret := m.Called()
+
+	r0 := ret.Get(0).([]cpu.CPUTimesStat)
+	r1 := ret.Error(1)
+
+	return r0, r1
+}