tests: use future::join instead of future::select (#2813)

* tests/av-store: use future::join instead of future::select

* tests/backing: use future::join instead of future::select

* tests/provisioner: use future::join instead of future::select

* tests/av-dist: use future::join instead of future::select

* tests/av-recovery: use future::join instead of future::select

* tests/bridge: use future::join instead of future::select

* tests/collator-protocol: use future::join instead of future::select

* tests/stmt-dist: use future::join instead of future::select

* fix tests