The spawned thread didn't have the tokio runtime active, which lead to
this error:
ERROR lsn_lease_bg_task{tenant_id=1bb647cb7d3974b52e74f7442fa7d059 timeline_id=cf41456d3202e1c3940cb8f372d160ab lsn=0/1576000}:panic{thread=<unnamed> location=compute_tools/src/lsn_lease.rs:201:5}: there is no reactor running, must be called from the context of a Tokio 1.x runtime
Fixes `test_readonly_node_gc`