chore: refactor notifier to use quartz.TickerFunc (#15134)

In investigating https://github.com/coder/internal/issues/109 I noticed many of the notification tests are still using `time.Sleep` and `require.Eventually`. This is an initial effort to start converting these to Quartz.

One product change is to switch the `notifier` to use a `TickerFunc` instead of a normal Ticker, since it allows the test to assert that a batch process is complete via the Quartz `Mock` clock.  This does introduce one slight behavioral change in that the notifier waits the fetch interval before processing its first batch.  In practice, this is inconsequential: no one will notice if we send notifications immediately on startup, or just a little later.

But, it does make a difference to some tests, which are fixed up here.

coderd/notifications/utils_test.go

@@ -92,3 +92,43 @@ func (i *dispatchInterceptor) Dispatcher(payload types.MessagePayload, title, bo
 		return retryable, err
 	}, nil
 }
+
+type dispatchCall struct {
+	payload     types.MessagePayload
+	title, body string
+	result      chan<- dispatchResult
+}
+
+type dispatchResult struct {
+	retryable bool
+	err       error
+}
+
+type chanHandler struct {
+	calls chan dispatchCall
+}
+
+func (c chanHandler) Dispatcher(payload types.MessagePayload, title, body string) (dispatch.DeliveryFunc, error) {
+	result := make(chan dispatchResult)
+	call := dispatchCall{
+		payload: payload,
+		title:   title,
+		body:    body,
+		result:  result,
+	}
+	return func(ctx context.Context, _ uuid.UUID) (bool, error) {
+		select {
+		case c.calls <- call:
+			select {
+			case r := <-result:
+				return r.retryable, r.err
+			case <-ctx.Done():
+				return false, ctx.Err()
+			}
+		case <-ctx.Done():
+			return false, ctx.Err()
+		}
+	}, nil
+}
+
+var _ notifications.Handler = &chanHandler{}