Remove deprecated usage of `tornado.IOLoop.run_sync`

[fjetter]

As one of the first steps of removing the tornado IOLoop from the code bas is to remove the usage of IOLoop.run_sync in favour of asyncio.run. The two have semantic differences and require us to touch the way we set up a few of our tests. The way tornado interacts with the event loop via IOLoop.run_sync is deprecated in CPython. This usage is connected to multiple warnings like "pending task while loop is closing", "task not awaited", etc.

Requirements

• Deprecated call to IOLoop.run_sync has been replaced by asyncio.run as appropriate

Implementation notes

• Test fixtures gen_cluster gen_test are using asyncio.run #6164
• No longer start stopped loops in LoopRunner to allow the use of asyncio.run #6163
• Remove all invocations of IOLoop.run_sync from CLI #6162

[fjetter]

Closing this. The remaining work can be done as part of #6163