Fix nginx-agent shutdown (#44)

ProcessWatcher waitgroup was already decremented in it's go routine so
the wg.Done() in plugin.Close() led to panic due to negative waitgroup.

This means on SIGTERM/SIGINT the main nginx-agent process would exit
with status code 2 which is reported in systemd status.

src/plugins/process_watcher.go

@@ -62,7 +62,7 @@ func (pw *ProcessWatcher) Info() *core.Info {
 
 func (pw *ProcessWatcher) Close() {
 	log.Info("ProcessWatcher is wrapping up")
-	pw.wg.Done()
+	pw.wg.Wait()
 }
 
 func (pw *ProcessWatcher) Process(message *core.Message) {}

src/plugins/process_watcher_test.go

@@ -155,9 +155,10 @@ func TestProcessWatcher_getProcUpdates(t *testing.T) {
 
 func TestProcessWatcher_Process(t *testing.T) {
 	env := tutils.GetMockEnvWithProcess()
-
 	pluginUnderTest := NewProcessWatcher(env, tutils.GetMockNginxBinary())
-	messagePipe := core.SetupMockMessagePipe(t, context.TODO(), pluginUnderTest)
+
+	ctx, cancel := context.WithCancel(context.TODO())
+	messagePipe := core.SetupMockMessagePipe(t, ctx, pluginUnderTest)
 
 	pluginUnderTest.Init(messagePipe)
 	messagePipe.Run()
@@ -173,6 +174,8 @@ func TestProcessWatcher_Process(t *testing.T) {
 		}
 	}
 
+	cancel()
+
 	pluginUnderTest.Close()
 }