test.py: s/asyncio.get_event_loop()/asyncio.get_running_loop()/

the latter raises a RuntimeError if there is no no running event loop,
while the former gets one from the the default policy in this case.
in the use cases in test.py, there is always a running event loop,
when `asyncio.get_event_loop()` gets called. so let's use
the preferred `asyncio.get_running_loop()`.

see https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.get_event_loop

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

Closes scylladb/scylladb#16398

test.py

@@ -1576,7 +1576,7 @@ async def main() -> int:
 
     signaled = asyncio.Event()
 
-    setup_signal_handlers(asyncio.get_event_loop(), signaled)
+    setup_signal_handlers(asyncio.get_running_loop(), signaled)
 
     try:
         await run_all_tests(signaled, options)
@@ -1616,10 +1616,10 @@ async def workaround_python26789() -> int:
     except (Exception, KeyboardInterrupt):
         def noop(x, y):
             return None
-        asyncio.get_event_loop().set_exception_handler(noop)
+        asyncio.get_running_loop().set_exception_handler(noop)
         traceback.print_exc()
         # Clear the custom handler
-        asyncio.get_event_loop().set_exception_handler(None)
+        asyncio.get_running_loop().set_exception_handler(None)
         return -1
     return code