add singlethread test

Author: fancaiyu

MCintegration/mc_multicpu_test.py

@@ -79,6 +79,9 @@ def two_integrands(x, f):
         if dist.is_initialized():
             dist.destroy_process_group()
 
+def test_mcmc_singlethread():
+    world_size = 1
+    init_process(rank=0, world_size=world_size, fn=run_mcmc, backend=backend)
 
 def test_mcmc(world_size=2):
     # Use fewer processes than CPU cores to avoid resource contention