Implement timeouts for all exec command runners

First is to write an internal CombinedOutput and Run function with a
timeout.

Second, the following instances of command runners need to have timeouts:

    plugins/inputs/ping/ping.go
    125:	out, err := c.CombinedOutput()

    plugins/inputs/exec/exec.go
    91:	if err := cmd.Run(); err != nil {

    plugins/inputs/ipmi_sensor/command.go
    31:	err := cmd.Run()

    plugins/inputs/sysstat/sysstat.go
    194:	out, err := cmd.CombinedOutput()

    plugins/inputs/leofs/leofs.go
    185:	defer cmd.Wait()

    plugins/inputs/sysstat/sysstat.go
    282:	if err := cmd.Wait(); err != nil {

closes #1067

internal/internal_test.go

@@ -1,6 +1,12 @@
 package internal
 
-import "testing"
+import (
+	"os/exec"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
 
 type SnakeTest struct {
 	input  string
@@ -30,3 +36,73 @@ func TestSnakeCase(t *testing.T) {
 		}
 	}
 }
+
+var (
+	sleepbin, _ = exec.LookPath("sleep")
+	echobin, _  = exec.LookPath("echo")
+)
+
+func TestRunTimeout(t *testing.T) {
+	if sleepbin == "" {
+		t.Skip("'sleep' binary not available on OS, skipping.")
+	}
+	cmd := exec.Command(sleepbin, "10")
+	start := time.Now()
+	err := RunTimeout(cmd, time.Millisecond*20)
+	elapsed := time.Since(start)
+
+	assert.Equal(t, TimeoutErr, err)
+	// Verify that command gets killed in 20ms, with some breathing room
+	assert.True(t, elapsed < time.Millisecond*75)
+}
+
+func TestCombinedOutputTimeout(t *testing.T) {
+	if sleepbin == "" {
+		t.Skip("'sleep' binary not available on OS, skipping.")
+	}
+	cmd := exec.Command(sleepbin, "10")
+	start := time.Now()
+	_, err := CombinedOutputTimeout(cmd, time.Millisecond*20)
+	elapsed := time.Since(start)
+
+	assert.Equal(t, TimeoutErr, err)
+	// Verify that command gets killed in 20ms, with some breathing room
+	assert.True(t, elapsed < time.Millisecond*75)
+}
+
+func TestCombinedOutput(t *testing.T) {
+	if echobin == "" {
+		t.Skip("'echo' binary not available on OS, skipping.")
+	}
+	cmd := exec.Command(echobin, "foo")
+	out, err := CombinedOutputTimeout(cmd, time.Second)
+
+	assert.NoError(t, err)
+	assert.Equal(t, "foo\n", string(out))
+}
+
+// test that CombinedOutputTimeout and exec.Cmd.CombinedOutput return
+// the same output from a failed command.
+func TestCombinedOutputError(t *testing.T) {
+	if sleepbin == "" {
+		t.Skip("'sleep' binary not available on OS, skipping.")
+	}
+	cmd := exec.Command(sleepbin, "foo")
+	expected, err := cmd.CombinedOutput()
+
+	cmd2 := exec.Command(sleepbin, "foo")
+	actual, err := CombinedOutputTimeout(cmd2, time.Second)
+
+	assert.Error(t, err)
+	assert.Equal(t, expected, actual)
+}
+
+func TestRunError(t *testing.T) {
+	if sleepbin == "" {
+		t.Skip("'sleep' binary not available on OS, skipping.")
+	}
+	cmd := exec.Command(sleepbin, "foo")
+	err := RunTimeout(cmd, time.Second)
+
+	assert.Error(t, err)
+}