rename build.{BlockDelay=>BlockDelaySecs}.

Since this global is not typed as a time.Duration,
rather as an int, it makes sense to clarify the unit.

node/modules/testing/beacon.go

@@ -8,5 +8,5 @@ import (
 )
 
 func RandomBeacon() (beacon.RandomBeacon, error) {
-	return beacon.NewMockBeacon(time.Duration(build.BlockDelay) * time.Second), nil
+	return beacon.NewMockBeacon(time.Duration(build.BlockDelaySecs) * time.Second), nil
 }