test(stages): fix deadlock with multiple static file writers

[shekhirin]

Fixes #7007

Opening multiple static files writers can cause a deadlock due to a shared underlying DashMap. If two static file segments get into the same shard, we deadlock.

➜  reth (alexey/stages-test-utils-deadlock) cargo test -p reth-stages --no-run
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.69s
  Executable unittests src/lib.rs (target/debug/deps/reth_stages-97c9591b70959ba5)

➜  reth (alexey/stages-test-utils-deadlock) for run in {1..50}; do target/debug/deps/reth_stages-97c9591b70959ba5 execute_intermediate_commit stage_checkpoint_pruned execute_sender_recovery | rg 'test result: ok'; done | wc -l
      50

[emhane]

so it was just bug in the test not in lib, good