*_test.go: fix test conditions

- add `testutil.RequiresRoot()` to TestMain
- moved `if testing.Short{ t.Skip() }` from each of the tests into a
common `newClient()`

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

checkpoint_test.go

@@ -6,13 +6,10 @@ import (
 )
 
 func TestCheckpointRestore(t *testing.T) {
-	if testing.Short() {
-		t.Skip()
-	}
 	if !supportsCriu {
 		t.Skip("system does not have criu installed")
 	}
-	client, err := New(address)
+	client, err := newClient(t, address)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -101,13 +98,10 @@ func TestCheckpointRestore(t *testing.T) {
 }
 
 func TestCheckpointRestoreNewContainer(t *testing.T) {
-	if testing.Short() {
-		t.Skip()
-	}
 	if !supportsCriu {
 		t.Skip("system does not have criu installed")
 	}
-	client, err := New(address)
+	client, err := newClient(t, address)
 	if err != nil {
 		t.Fatal(err)
 	}