functional: always use list-unit-files to fetch a units list

For some reason "fleetctl list-units" occasionally returns a list
without some entries, especially when loading multiple units at the
same time. Until the issue gets resolved, let's avoid the such errors,
by using "fleetctl list-unit-files".

functional/unit_action_test.go

@@ -354,13 +354,6 @@ func replaceUnitMultiple(cmd string, n int) error {
 		return fmt.Errorf("invalid command %s", cmd)
 	}
 
-	var listUnitCmd string
-	if cmd == "submit" {
-		listUnitCmd = "list-unit-files"
-	} else {
-		listUnitCmd = "list-units"
-	}
-
 	cluster, err := platform.NewNspawnCluster("smoke")
 	if err != nil {
 		return fmt.Errorf("%v", err)
@@ -396,9 +389,9 @@ func replaceUnitMultiple(cmd string, n int) error {
 			return fmt.Errorf("Unable to %s fleet unit: %v", cmd, err)
 		}
 
-		stdout, _, err = cluster.Fleetctl(m, listUnitCmd, "--no-legend")
+		stdout, _, err = cluster.Fleetctl(m, "list-unit-files", "--no-legend")
 		if err != nil {
-			return fmt.Errorf("Failed to run %s: %v", listUnitCmd, err)
+			return fmt.Errorf("Failed to run %s: %v", "list-unit-files", err)
 		}
 		units := strings.Split(strings.TrimSpace(stdout), "\n")
 		if len(units) != i {
@@ -419,9 +412,9 @@ func replaceUnitMultiple(cmd string, n int) error {
 		if _, _, err = cluster.Fleetctl(m, cmd, "--replace", curHelloService); err != nil {
 			return fmt.Errorf("Unable to replace fleet unit: %v", err)
 		}
-		stdout, _, err = cluster.Fleetctl(m, listUnitCmd, "--no-legend")
+		stdout, _, err = cluster.Fleetctl(m, "list-unit-files", "--no-legend")
 		if err != nil {
-			return fmt.Errorf("Failed to run %s: %v", listUnitCmd, err)
+			return fmt.Errorf("Failed to run %s: %v", "list-unit-files", err)
 		}
 		units := strings.Split(strings.TrimSpace(stdout), "\n")
 		if len(units) != n {