[perf] NoOpTest.run_async() should yield to avoid blocking event loop (Azure#23741)

- Fixes Azure#23740
- Reduces throughput by almost a factor of 10 (1.3M vs 160k ops/sec), but allows for parallel execution and should more accurately represent real-world async performance.

Author: mikeharder

tools/azure-devtools/src/azure_devtools/perfstress_tests/system_perfstress/no_op_test.py

@@ -3,6 +3,7 @@
 # Licensed under the MIT License. See License.txt in the project root for license information.
 # --------------------------------------------------------------------------------------------
 
+import asyncio
 from azure_devtools.perfstress_tests import PerfStressTest
 
 
@@ -11,4 +12,7 @@ def run_sync(self):
         pass
 
     async def run_async(self):
-        pass
+        # Yield on each call to avoid blocking the event loop.  Also simulates a real call to async IO.
+        # Reduces throughput by almost a factor of 10 (1.3M vs 160k ops/sec), but allows for parallel execution
+        # and should more accurately represent real-world async performance.
+        await asyncio.sleep(0)