pytests: xpay calling askrene-age causes BROKEN line during teardown

[daywalker90]

It happens quite frequently that during teardown cln-askrene exits first, then xpay's task of calling askrene-age fires but does not find the command and logs a BROKEN line and then xpay exits. This causes the teardown error, failing the test unnecessarily.

--------------------------- Captured stdout teardown ---------------------------
lightningd-1 2026-06-09T13:10:53.823Z UNUSUAL lightningd: JSON-RPC shutdown
lightningd-1 2026-06-09T13:10:53.836Z DEBUG   lightningd: io_break: start_json_stream
lightningd-1 2026-06-09T13:10:53.836Z DEBUG   lightningd: io_loop_with_timers: main
lightningd-1 2026-06-09T13:10:53.845Z DEBUG   connectd: REPLY WIRE_CONNECTD_START_SHUTDOWN_REPLY with 0 fds
lightningd-1 2026-06-09T13:10:53.845Z DEBUG   lightningd: io_break: connectd_start_shutdown_reply
lightningd-1 2026-06-09T13:10:53.858Z DEBUG   033845802d25b4e074ccfd7cd8b339a41dc75bf9978a034800444b51d42b07799a-onchaind-chan#1: Status closed, but not exited. Killing
lightningd-1 2026-06-09T13:10:53.894Z DEBUG   plugin-cln-askrene: Killing plugin: exited during normal operation
lightningd-1 2026-06-09T13:10:53.898Z DEBUG   plugin-spenderp: Killing plugin: exited during normal operation
lightningd-1 2026-06-09T13:10:53.899Z DEBUG   plugin-sql: Killing plugin: exited during normal operation
lightningd-1 2026-06-09T13:10:53.906Z DEBUG   plugin-bookkeeper: Killing plugin: exited during normal operation
lightningd-1 2026-06-09T13:10:53.957Z **BROKEN** plugin-cln-xpay: askrene-age failed with {"code":-32601,"message":"Unknown command 'askrene-age'"}
lightningd-1 2026-06-09T13:10:53.964Z DEBUG   plugin-cln-xpay: Killing plugin: exited during normal operation
lightningd-1 2026-06-09T13:10:53.970Z DEBUG   plugin-keysend: Killing plugin: exited during normal operation
lightningd-1 2026-06-09T13:10:53.970Z DEBUG   plugin-offers: Killing plugin: exited during normal operation