Basic inbox test (#100)

* Add tests for Scheduler and Inbox in actor package

new tests for the Scheduler and Inbox functionality. It's not much, but it's a start.

* replace sleep with Gosched. Shave a millisecond off the test time. :-)

* nitpick.

actor/inbox_test.go

@@ -1 +1,57 @@
 package actor
+
+import (
+	"runtime"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func TestScheduler(t *testing.T) {
+	var executed atomic.Bool
+	scheduler := NewScheduler(10)
+	scheduler.Schedule(func() {
+		executed.Store(true)
+	})
+	runtime.Gosched()
+	if !executed.Load() {
+		t.Errorf("Expected the function to be executed")
+	}
+}
+
+func TestInboxSendAndProcess(t *testing.T) {
+	inbox := NewInbox(10)
+	processedMessages := make(chan Envelope, 10)
+	mockProc := MockProcesser{
+		processFunc: func(envelopes []Envelope) {
+			for _, e := range envelopes {
+				processedMessages <- e
+			}
+		},
+	}
+	inbox.Start(mockProc)
+	msg := Envelope{}
+	inbox.Send(msg)
+	select {
+	case <-processedMessages: // Message processed
+	case <-time.After(time.Millisecond):
+		t.Errorf("Message was not processed in time")
+	}
+
+	inbox.Stop()
+}
+
+type MockProcesser struct {
+	processFunc func([]Envelope)
+}
+
+func (m MockProcesser) Start() {}
+func (m MockProcesser) PID() *PID {
+	return nil
+}
+func (m MockProcesser) Send(*PID, any, *PID) {}
+func (m MockProcesser) Invoke(envelopes []Envelope) {
+	m.processFunc(envelopes)
+}
+func (m MockProcesser) Shutdown(_ *sync.WaitGroup) {}