Enhance TestValidatingBatchingSinkPrepare (#2705)

The test now actually validates that processing the provided
hints populates the VBS' chunk cache

go/types/validating_batching_sink_test.go

@@ -28,7 +28,9 @@ func TestValidatingBatchingSinkPrepare(t *testing.T) {
 
 	vbs := NewValidatingBatchingSink(cs)
 	vbs.Prepare(hints)
-	assert.Equal(t, 5, cs.Reads)
+	for h := range hints {
+		vbs.vs.isPresent(h)
+	}
 }
 
 func TestValidatingBatchingSinkDecode(t *testing.T) {