Setup for stateful test of txpool

[jomuel]

Remaining rules to draw input data and model txpool's behavior will be added in the next PR.

[AskAlexSharov]

@jomuel hi. need to merge "main" and run make test

[AlexeyAkhunov]

I think you need to do cleanup for db transactions (Rollback) and the databases (Close) using defer otherwise the tests hit the limit of open RO transactions. Also, it is better to create temporary directories using functions of the type testing.T, because these are automatically cleaned up on exit