Bug: sporadic scheduling does not work for multiple threads

Hi,

I am using NuttX for micro-ROS on STM32 microcontroller on Olimex board. [Link to example](https://github.com/micro-ROS/nuttx_apps/blob/feature/foxy_rbs_one_sporadic_thread/examples/uros_rbs/main_rbs.c)

which is a very simple extension of the [NuttX example for sporadic scheduling in testing/ostest](https://github.com/micro-ROS/nuttx_apps/blob/feature/foxy_rbs_one_sporadic_thread/testing/ostest/sporadic.c)

**Test setup:**
- [sporadic thread 1 configuration: 20ms/100ms budget/period](https://github.com/micro-ROS/nuttx_apps/blob/6f692d9bfcb0e3d1971e2b2a6448d1084d5a81ea/examples/uros_rbs/main_rbs.c#L367)
- [sporadic thread 2 configuration 10ms/100ms budget/period](https://github.com/micro-ROS/nuttx_apps/blob/6f692d9bfcb0e3d1971e2b2a6448d1084d5a81ea/examples/uros_rbs/main_rbs.c#L399)
- the callback function [increments a counter every millisecond](https://github.com/micro-ROS/nuttx_apps/blob/6f692d9bfcb0e3d1971e2b2a6448d1084d5a81ea/examples/uros_rbs/main_rbs.c#L174).
- [wait for 10 seconds](https://github.com/micro-ROS/nuttx_apps/blob/6f692d9bfcb0e3d1971e2b2a6448d1084d5a81ea/examples/uros_rbs/main_rbs.c#L464)
- output the [number of ms-counters](https://github.com/micro-ROS/nuttx_apps/blob/6f692d9bfcb0e3d1971e2b2a6448d1084d5a81ea/examples/uros_rbs/main_rbs.c#L476) for every sporadic thread

**Observation:**
- When I configure the application with one sporadic thread and one FIFO thread (with lower priority and 100% CPU utilization), budget enforcement works well.
- When I configure the application with two sporadic threads and one FIFO thread (with lower priority and 100% CPU utilization) budget enforcement does not work any more. I tested the sporadic scheduling with different budget/period configurations: 10ms/100ms and 1second/10seconds (the internal tick - conversion was correct).
- I saw also somewhere the requirement that the buget shall be at most half of the period, so I configured the budget always smaller then half of the period for all individual threads:
- thread 1: 20ms/100ms
- thread 2: 10ms/100ms

**Problem description**
I came to the conclusion, that budget enforcement of the NuttX sporadic scheduling only works for one sporadic thread.
For real applications, I would like to use multiple threads with sporadic scheduling.

Could you please check the implementation and give support?