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.

Fixes CI failures on Arm's FVP SMP configurations.

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