move legacy test for delta function

promql/testdata/functions.test

@@ -85,6 +85,15 @@ eval instant at 30m irate(http_requests[50m])
 
 clear
 
+# Tests for delta().
+load 5m
+	http_requests{path="/foo"}	0 50 100 150 200
+
+eval instant at 20m delta(http_requests[20m])
+	{path="/foo"} 200
+
+clear
+
 # Tests for deriv() and predict_linear().
 load 5m
 	testcounter_reset_middle	0+10x4 0+10x5

promql/testdata/legacy.test

@@ -91,11 +91,6 @@ eval instant at 50m x{y="testvalue"}
 	x{y="testvalue"} 100
 
 
-# Deltas should be adjusted for target interval vs. samples under target interval.
-eval instant at 50m delta(http_requests{group="canary", instance="1", job="app-server"}[18m])
-	{group="canary", instance="1", job="app-server"} 288
-
-
 # Rates should calculate per-second rates.
 eval instant at 50m rate(http_requests{group="canary", instance="1", job="app-server"}[50m])
 	{group="canary", instance="1", job="app-server"} 0.26666666666666666