Add test case for labels attached to servergroups

Author: jacksontj

test/testdata/selectors.test

@@ -34,6 +34,9 @@ eval instant at 18000s rate(http_requests{group=~".*ry", instance="1"}[1m])
 	{az="a", job="api-server", instance="1", group="canary"} 4
 	{az="b", job="api-server", instance="1", group="canary"} 4
 
+eval instant at 18000s rate(http_requests{group=~".*ry", instance="1", az="a"}[1m])
+	{az="a", job="api-server", instance="1", group="canary"} 4
+
 eval instant at 18000s rate(http_requests{instance!="3"}[1m] offset 10000s)
 	{az="a", job="api-server", instance="0", group="production"} 1
 	{az="a", job="api-server", instance="1", group="production"} 2