ci: fix TestNilResources when systemd not available

Split the test into two -- for fs and systemd cgroup managers, and only
run the second one if systemd is available.

Prevents the following failure during `make unittest`:

> === RUN   TestNilResources
>     manager_test.go:27: systemd not running on this host, cannot use systemd cgroups manager
> --- FAIL: TestNilResources (0.22s)

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/cgroups/manager/manager_test.go

@@ -3,6 +3,7 @@ package manager
 import (
 	"testing"
 
+	"github.com/opencontainers/runc/libcontainer/cgroups/systemd"
 	"github.com/opencontainers/runc/libcontainer/configs"
 )
 
@@ -10,35 +11,45 @@ import (
 // config.Resources is nil. While it does not make sense to use a
 // manager with no resources, it should not result in a panic.
 //
-// This tests either v1 or v2 managers (both fs and systemd),
-// depending on what cgroup version is available on the host.
+// This tests either v1 or v2 fs cgroup manager, depending on which
+// cgroup version is available.
 func TestNilResources(t *testing.T) {
-	for _, sd := range []bool{false, true} {
-		cg := &configs.Cgroup{} // .Resources is nil
-		cg.Systemd = sd
-		mgr, err := New(cg)
-		if err != nil {
-			// Some managers require non-nil Resources during
-			// instantiation -- provide and retry. In such case
-			// we're mostly testing Set(nil) below.
-			cg.Resources = &configs.Resources{}
-			mgr, err = New(cg)
-			if err != nil {
-				t.Error(err)
-				continue
-			}
-		}
-		_ = mgr.Apply(-1)
-		_ = mgr.Set(nil)
-		_ = mgr.Freeze(configs.Thawed)
-		_ = mgr.Exists()
-		_, _ = mgr.GetAllPids()
-		_, _ = mgr.GetCgroups()
-		_, _ = mgr.GetFreezerState()
-		_ = mgr.Path("")
-		_ = mgr.GetPaths()
-		_, _ = mgr.GetStats()
-		_, _ = mgr.OOMKillCount()
-		_ = mgr.Destroy()
-	}
+	testNilResources(t, false)
+}
+
+// TestNilResourcesSystemd is the same as TestNilResources,
+// only checking the systemd cgroup manager.
+func TestNilResourcesSystemd(t *testing.T) {
+	if !systemd.IsRunningSystemd() {
+		t.Skip("requires systemd")
+	}
+	testNilResources(t, true)
+}
+
+func testNilResources(t *testing.T, systemd bool) {
+	cg := &configs.Cgroup{} // .Resources is nil
+	cg.Systemd = systemd
+	mgr, err := New(cg)
+	if err != nil {
+		// Some managers require non-nil Resources during
+		// instantiation -- provide and retry. In such case
+		// we're mostly testing Set(nil) below.
+		cg.Resources = &configs.Resources{}
+		mgr, err = New(cg)
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+	_ = mgr.Apply(-1)
+	_ = mgr.Set(nil)
+	_ = mgr.Freeze(configs.Thawed)
+	_ = mgr.Exists()
+	_, _ = mgr.GetAllPids()
+	_, _ = mgr.GetCgroups()
+	_, _ = mgr.GetFreezerState()
+	_ = mgr.Path("")
+	_ = mgr.GetPaths()
+	_, _ = mgr.GetStats()
+	_, _ = mgr.OOMKillCount()
+	_ = mgr.Destroy()
 }