internal/testutil: create, add SkipOnCentOS

CentOS 7 is showing its age and we'd rather skip some tests on it than
find out why they are flaky.

Add internal/testutil package, and move the generalized version of
SkipOnCentOS7 from libcontainer/cgroups/devices to there.

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

internal/testutil/testutil.go

@@ -0,0 +1,30 @@
+package testutil
+
+import (
+	"os/exec"
+	"strconv"
+	"sync"
+	"testing"
+)
+
+var (
+	centosVer     string
+	centosVerOnce sync.Once
+)
+
+func centosVersion() string {
+	centosVerOnce.Do(func() {
+		ver, _ := exec.Command("rpm", "-q", "--qf", "%{version}", "centos-release").CombinedOutput()
+		centosVer = string(ver)
+	})
+	return centosVer
+}
+
+func SkipOnCentOS(t *testing.T, reason string, versions ...int) {
+	t.Helper()
+	for _, v := range versions {
+		if vstr := strconv.Itoa(v); centosVersion() == vstr {
+			t.Skip(reason + " on CentOS " + vstr)
+		}
+	}
+}