Validate timeUnit in ramping-arrival-rate

Previous to this negative time units will error but 0 timeunit will
panic.

This was fixed in constant-arrival-rate long ago, but nobody checked
ramping-arrival-rate.

lib/executor/executors_test.go

@@ -340,6 +340,7 @@ var configMapTestCases = []configMapTestCase{
 	},
 	{`{"carrival": {"executor": "constant-arrival-rate", "rate": 10, "duration": "10m", "preAllocatedVUs": 20, "maxVUs": 30}}`, exp{}},
 	{`{"carrival": {"executor": "constant-arrival-rate", "rate": 10, "duration": "10m", "preAllocatedVUs": 20, "maxVUs": 30, "timeUnit": "-1s"}}`, exp{validationError: true}},
+	{`{"carrival": {"executor": "constant-arrival-rate", "rate": 10, "duration": "10m", "preAllocatedVUs": 20, "maxVUs": 30, "timeUnit": "0s"}}`, exp{validationError: true}},
 	{
 		`{"carrival": {"executor": "constant-arrival-rate", "rate": 10, "duration": "10m", "preAllocatedVUs": 20}}`,
 		exp{custom: func(t *testing.T, cm lib.ScenarioConfigs) {
@@ -403,6 +404,7 @@ var configMapTestCases = []configMapTestCase{
 	{`{"varrival": {"executor": "ramping-arrival-rate", "preAllocatedVUs": 20, "maxVUs": 50}}`, exp{validationError: true}},
 	{`{"varrival": {"executor": "ramping-arrival-rate", "preAllocatedVUs": 20, "maxVUs": 50, "stages": []}}`, exp{validationError: true}},
 	{`{"varrival": {"executor": "ramping-arrival-rate", "preAllocatedVUs": 20, "maxVUs": 50, "stages": [{"duration": "5m", "target": 10}], "timeUnit": "-1s"}}`, exp{validationError: true}},
+	{`{"varrival": {"executor": "ramping-arrival-rate", "preAllocatedVUs": 20, "maxVUs": 50, "stages": [{"duration": "5m", "target": 10}], "timeUnit": "0s"}}`, exp{validationError: true}},
 	{`{"varrival": {"executor": "ramping-arrival-rate", "preAllocatedVUs": 30, "maxVUs": 20, "stages": [{"duration": "5m", "target": 10}]}}`, exp{validationError: true}},
 	// TODO: more tests of mixed executors and execution plans
 

lib/executor/ramping_arrival_rate.go

@@ -91,7 +91,7 @@ func (varc *RampingArrivalRateConfig) Validate() []error {
 		errors = append(errors, fmt.Errorf("the startRate value can't be negative"))
 	}
 
-	if varc.TimeUnit.TimeDuration() < 0 {
+	if varc.TimeUnit.TimeDuration() <= 0 {
 		errors = append(errors, fmt.Errorf("the timeUnit must be more than 0"))
 	}