daemon: FakeCommand usage requires reaper.Start()

The usage of testutil.FakeCommand requires the test environment to start
and stop the reaper in the case where the reaper option is enabled.

For example, in internals/daemon/daemon_test.go:

:
cmd := testutil.FakeCommand(c, "shutdown", "", true)
:

However, in daemon_test.go, the code is currently relying on the service
manager, provided by the overlord, to start the reaper. This is not a
safe solution as not all test implementations may actually run the real
overlord code, and even if they do, we have a potential race condition.

daemon.Init() -> overlord.New() -> servstate.NewManager() -> reaper.Start()

The following changes are introduced:

- Add reaper.Start() and reaper.Stop() to the daemon test setup and teardown.

- Add a reaper based test for testutil.FakeCommand().

internals/daemon/daemon_test.go

@@ -41,6 +41,7 @@ import (
 	"github.com/canonical/pebble/internals/overlord/restart"
 	"github.com/canonical/pebble/internals/overlord/standby"
 	"github.com/canonical/pebble/internals/overlord/state"
+	"github.com/canonical/pebble/internals/reaper"
 	"github.com/canonical/pebble/internals/systemd"
 	"github.com/canonical/pebble/internals/testutil"
 )
@@ -62,6 +63,8 @@ type daemonSuite struct {
 var _ = check.Suite(&daemonSuite{})
 
 func (s *daemonSuite) SetUpTest(c *check.C) {
+	err := reaper.Start()
+	c.Assert(err, check.IsNil)
 	s.pebbleDir = c.MkDir()
 	s.statePath = filepath.Join(s.pebbleDir, ".pebble.state")
 	systemdSdNotify = func(notif string) error {
@@ -75,6 +78,8 @@ func (s *daemonSuite) TearDownTest(c *check.C) {
 	s.notified = nil
 	s.authorized = false
 	s.err = nil
+	err := reaper.Stop()
+	c.Assert(err, check.IsNil)
 }
 
 func (s *daemonSuite) newDaemon(c *check.C) *Daemon {

internals/testutil/exec_test.go

@@ -21,12 +21,24 @@ import (
 	"path/filepath"
 
 	"gopkg.in/check.v1"
+
+	"github.com/canonical/pebble/internals/reaper"
 )
 
 type fakeCommandSuite struct{}
 
 var _ = check.Suite(&fakeCommandSuite{})
 
+func (s *fakeCommandSuite) SetUpSuite(c *check.C) {
+	err := reaper.Start()
+	c.Assert(err, check.IsNil)
+}
+
+func (s *fakeCommandSuite) TearDownSuite(c *check.C) {
+	err := reaper.Stop()
+	c.Assert(err, check.IsNil)
+}
+
 func (s *fakeCommandSuite) TestFakeCommand(c *check.C) {
 	fake := FakeCommand(c, "cmd", "true", false)
 	defer fake.Restore()
@@ -40,6 +52,13 @@ func (s *fakeCommandSuite) TestFakeCommand(c *check.C) {
 	})
 }
 
+func (s *fakeCommandSuite) TestFakeCommandWithReaper(c *check.C) {
+	fake := FakeCommand(c, "cmd", "true", true)
+	defer fake.Restore()
+	err := exec.Command("cmd", "").Run()
+	c.Assert(err, check.IsNil)
+}
+
 func (s *fakeCommandSuite) TestFakeCommandAlso(c *check.C) {
 	fake := FakeCommand(c, "fst", "", false)
 	also := fake.Also("snd", "")