samples: zbus: benchmark: Filter SMP from benchmark_sync test

The benchmark_sync test produces incorrect results on SMP systems due to
a race condition between the producer thread and subscriber threads that
only occurs with parallel execution.

Thread configuration:
- Producer thread: priority 5 (lower priority, runs later)
- Subscriber threads (8): priority 3 (higher priority, runs first)

Expected behavior on uniprocessor:
  1. Producer publishes message
  2. Subscriber immediately preempts producer (priority 3 < 5)
  3. Subscriber processes message, calls atomic_add(&count, 256)
  4. Producer resumes, continues to next message

  Result: All messages counted before producer checks final count.

Race condition on SMP:
  CPU 0 (Producer):              CPU 1-3 (Subscribers):
  -----------------              ----------------------
  Publish msg 1
  k_msgq_put(sub1) ✓
                                 k_msgq_get() → processing
  Publish msg 2
  k_msgq_put(sub2) ✓
                                 k_msgq_get() → processing
  ...continues...
  Publish msg 128 ✓
  atomic_get(&count)             Still processing...
  → Reports incomplete count!    atomic_add() comes later

On SMP, the producer doesn't get preempted since it runs on a different
CPU from the subscribers. It races ahead and checks atomic_get(&count)
while subscriber threads on other CPUs are still processing messages.

Observed results:
- Non-SMP: Bytes sent = 262144, received = 262144 ✓
- SMP: Bytes sent = 262144, received = 260352 ✗ (7 messages lost)

This is a benchmark test design issue, not a zbus bug. The test assumes
subscribers complete before the producer finishes, which doesn't hold on
SMP systems. Filter SMP configurations from this test variant for now.

Signed-off-by: Nicolas Pitre <[email protected]>

Author: Nicolas Pitre

samples/subsys/zbus/benchmark/sample.yaml

@@ -46,7 +46,10 @@ tests:
   sample.zbus.benchmark_sync:
     tags: zbus
     min_ram: 16
-    filter: CONFIG_SYS_CLOCK_EXISTS and not (CONFIG_ARCH_POSIX and not CONFIG_BOARD_NATIVE_SIM)
+    filter: >-
+      CONFIG_SYS_CLOCK_EXISTS and
+      not (CONFIG_ARCH_POSIX and not CONFIG_BOARD_NATIVE_SIM) and
+      not CONFIG_SMP
     harness: console
     harness_config:
       type: multi_line