Flaky transaction pool test

[bkchr]

 ---- testing::pool::should_push_watchers_during_maintaince stdout ----
thread 'testing::pool::should_push_watchers_during_maintaince' panicked at 'assertion failed: `(left == right)`
  left: `5`,
 right: `3`', client/transaction-pool/src/testing/pool.rs:331:2

Job: https://gitlab.parity.io/parity/substrate/-/jobs/407602

cc @tomusdrw @NikVolf

[kianenigma]

Sorry, but happens again :(

https://gitlab.parity.io/parity/substrate/-/jobs/440163

cc @NikVolf

[kianenigma]

https://gitlab.parity.io/parity/substrate/-/jobs/441783
https://giphy.com/gifs/Friends-episode-4-friends-tv-the-one-with-rachels-assistant-StoeNoDkYuum8cj8MV

(it is a different test case though)