PromQL: Test negative offset in PromQL tests

Signed-off-by: beorn7 <beorn@grafana.com>

promql/test.go

@@ -613,6 +613,7 @@ func (t *Test) clear() {
 		Timeout:                  100 * time.Second,
 		NoStepSubqueryIntervalFn: func(int64) int64 { return durationMilliseconds(1 * time.Minute) },
 		EnableAtModifier:         true,
+		EnableNegativeOffset:     true,
 	}
 
 	t.queryEngine = NewEngine(opts)

promql/testdata/at_modifier.test

@@ -18,6 +18,14 @@ eval instant at 10s metric offset 50s @ 100
   metric{job="1"} 5
   metric{job="2"} 10
 
+eval instant at 10s metric @ 0 offset -50s
+  metric{job="1"} 5
+  metric{job="2"} 10
+
+eval instant at 10s metric offset -50s @ 0
+  metric{job="1"} 5
+  metric{job="2"} 10
+
 eval instant at 10s -metric @ 100
   {job="1"} -10
   {job="2"} -20

promql/testdata/selectors.test

@@ -29,6 +29,12 @@ eval instant at 18000s rate(http_requests{instance!="3"}[1m] offset 10000s)
 	{job="api-server", instance="0", group="canary"} 3
 	{job="api-server", instance="1", group="canary"} 4
 
+eval instant at 4000s rate(http_requests{instance!="3"}[1m] offset -4000s)
+	{job="api-server", instance="0", group="production"} 1
+	{job="api-server", instance="1", group="production"} 2
+	{job="api-server", instance="0", group="canary"} 3
+	{job="api-server", instance="1", group="canary"} 4
+
 eval instant at 18000s rate(http_requests[40s]) - rate(http_requests[1m] offset 10000s)
 	{job="api-server", instance="0", group="production"} 2
 	{job="api-server", instance="1", group="production"} 1