[utest]:Modify the execution logic of smp_thread_preemptions

Currently, the print information of this test case fails to demonstrate that
the high-priority thread has preempted the low-priority thread. This is because
when the high-priority thread prints the thread list, there is no information
about the low-priority thread (tlow), as tlow has already completed execution
and been destroyed. Therefore, the current execution logic cannot confirm the
successful completion of the preemption operation.

Solution: After the low-priority thread (tlow) releases the lock, add a busy-wait
loop while(!finish_flag);. At this point, when the high-priority thread (thigh)
prints the thread list information, tlow can be observed in the ready state, indicating
that it has been preempted by thigh.

Signed-off-by: Mengchen Teng <teng_mengchen@163.com>

src/utest/smp/smp_thread_preemption_tc.c

@@ -17,6 +17,7 @@
  * @note    Create multiple threads, low-priority threads run first,
  *          high-priority threads preempt low-priority threads, and
  *          print the current status of each core in the thread's entry function.
+ *
  */
 
 #define THREAD_PRIORITY_HIGH 21
@@ -24,6 +25,7 @@
 #define THREAD_STACK_SIZE UTEST_THR_STACK_SIZE
 
 static rt_thread_t   threads[2];
+static volatile rt_uint8_t    finish_flag = 0;
 static struct rt_spinlock lock;
 
 /* High Priority Thread */
@@ -35,6 +37,7 @@ static void thread_high_entry(void *parameter)
     extern long list_thread(void);
     list_thread();
     rt_spin_unlock(&lock);
+    finish_flag = 1;
 }
 
 /* Low Priority Thread */
@@ -46,6 +49,7 @@ static void thread_low_entry(void *parameter)
     extern long list_thread(void);
     list_thread();
     rt_spin_unlock(&lock);
+    while (!finish_flag);
 }
 
 static void thread_preemptions_tc(void)
@@ -72,6 +76,7 @@ static void thread_preemptions_tc(void)
 static rt_err_t utest_tc_init(void)
 {
     rt_spin_lock_init(&lock);
+    finish_flag = 0;
     return RT_EOK;
 }