functional: exclude ActiveEnterTimestamp from unit comparison condition

Actually it's not necessary to check that both units have the same value
for ActiveEnterTimestamp, as it can vary at any time. So let's exclude
ActiveEnterTimestamp from the condition for unit comparison.

functional/systemd_test.go

@@ -119,8 +119,19 @@ func waitForUnitState(mgr unit.UnitManager, name string, want unit.UnitState) er
 			return err
 		}
 
-		if reflect.DeepEqual(want, *got) {
+		if isEqualUnitState(want, *got) {
 			return nil
 		}
 	}
 }
+
+// isEqualUnitState checks if both units are the same,
+// excluding ActiveEnterTimestamp field of each unit state.
+func isEqualUnitState(src, dst unit.UnitState) bool {
+	return src.LoadState == dst.LoadState &&
+		src.ActiveState == dst.ActiveState &&
+		src.SubState == dst.SubState &&
+		src.MachineID == dst.MachineID &&
+		src.UnitHash == dst.UnitHash &&
+		src.UnitName == dst.UnitName
+}