Remove transaction-pool `test-helpers` feature (#10571)

* Remove transaction-pool `test-helpers` feature

`test-helpers` feature is a bad idea in general, because once the feature is enabled somewhere in
the workspace, it is enabled anywhere. While removing the feature, the tests were also rewritten to
get rid off other "only test" related code.

Contributes towards: https://github.com/paritytech/substrate/issues/9727



* Apply suggestions from code review

Co-authored-by: André Silva <[email protected]>

* Fix benches

Co-authored-by: André Silva <[email protected]>