Refactor process package and introduce ProcessManager{} with tests (#75)

* Add a process.Manager singleton with process.GetManager()

* Use process.GetManager everywhere

* Fix godoc comments for process module

* Increment process counter id after locking the mutex

modules/process/manager_test.go

@@ -0,0 +1,33 @@
+package process
+
+import (
+	"os/exec"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestManager_Add(t *testing.T) {
+	pm := Manager{Processes: make(map[int64]*Process)}
+
+	pid := pm.Add("foo", exec.Command("foo"))
+	assert.Equal(t, int64(1), pid, "expected to get pid 1 got %d", pid)
+
+	pid = pm.Add("bar", exec.Command("bar"))
+	assert.Equal(t, int64(2), pid, "expected to get pid 2 got %d", pid)
+}
+
+func TestManager_Remove(t *testing.T) {
+	pm := Manager{Processes: make(map[int64]*Process)}
+
+	pid1 := pm.Add("foo", exec.Command("foo"))
+	assert.Equal(t, int64(1), pid1, "expected to get pid 1 got %d", pid1)
+
+	pid2 := pm.Add("bar", exec.Command("bar"))
+	assert.Equal(t, int64(2), pid2, "expected to get pid 2 got %d", pid2)
+
+	pm.Remove(pid2)
+
+	_, exists := pm.Processes[pid2]
+	assert.False(t, exists, "PID %d is in the list but shouldn't", pid2)
+}