`tests::test_not_pruning_network_graph_until_graph_sync_completion` looks unstable

[TheBlueMatt]

CI on #1544 failed with the following, implying the test is not stable:

     Running `/Users/runner/work/rust-lightning/rust-lightning/target/debug/deps/lightning_background_processor-7b9bd717de8cc6ab`
running 9 tests
test tests::test_channel_manager_persist_error ... ok
test tests::test_invoice_payer ... ok
test tests::test_network_graph_persist_error ... ok
test tests::test_background_event_handling ... ok
test tests::test_scorer_persist_error ... ok
test tests::test_scorer_persistence ... ok
test tests::test_not_pruning_network_graph_until_graph_sync_completion ... FAILED
test tests::test_background_processor ... ok
test tests::test_timer_tick_called ... ok
failures:
---- tests::test_not_pruning_network_graph_until_graph_sync_completion stdout ----
DEBUG node 0 [lightning::ln::channelmanager : lightning/src/ln/channelmanager.rs, 5921] Generating channel_reestablish events for 0355f8d2238a322d16b602bd0ceaad5b01019fb055971eaadcc9b29226a4da6c23
DEBUG node 1 [lightning::ln::channelmanager : lightning/src/ln/channelmanager.rs, 5921] Generating channel_reestablish events for 027f921585f2ac0c7c70e36110adecfd8fd14b8a99bfb3d000a283fcac358fce88
TRACE node 0 [lightning_background_processor : lightning-background-processor/src/lib.rs, 269] Calling ChannelManager's timer_tick_occurred on startup
TRACE node 0 [lightning_background_processor : lightning-background-processor/src/lib.rs, 303] Persisting ChannelManager...
TRACE node 0 [lightning_background_processor : lightning-background-processor/src/lib.rs, 305] Done persisting ChannelManager.
TRACE node 0 [lightning_background_processor : lightning-background-processor/src/lib.rs, 313] Calling ChannelManager's timer_tick_occurred
TRACE node 0 [lightning_background_processor : lightning-background-processor/src/lib.rs, 334] Calling PeerManager's timer_tick_occurred
TRACE node 0 [lightning_background_processor : lightning-background-processor/src/lib.rs, 345] Assessing prunability of network graph
TRACE node 0 [lightning_background_processor : lightning-background-processor/src/lib.rs, 356] Not pruning network graph, either due to pending rapid gossip sync or absence of a prunable graph.
TRACE node 0 [lightning_background_processor : lightning-background-processor/src/lib.rs, 362] Persisting scorer
thread 'tests::test_not_pruning_network_graph_until_graph_sync_completion' panicked at 'Network graph not pruned within deadline: Timeout', lightning-background-processor/src/lib.rs:966:14
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
TRACE node 0 [lightning_background_processor : lightning-background-processor/src/lib.rs, 309] Terminating background processor.
Failed to remove test persister directory test_not_pruning_network_graph_until_graph_sync_completion_persister_1: No such file or directory (os error 2)
failures:
    tests::test_not_pruning_network_graph_until_graph_sync_completion
test result: FAILED. 8 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 21.65s

[TheBlueMatt]

Oh, it failed on macos-latest.