New testing helpers (#6555)

* Initial commit

Forked at: 8ef1ac0e
Parent branch: origin/master

* Add send_transaction to RpcHandlers

* Extension trait for RpcHandlers

* Revert "Add send_transaction to RpcHandlers"

This reverts commit 03c89e13

.

* Add an extension trait for BlockchainEvents

* Update test-utils/client/src/lib.rs

Co-authored-by: Bastian Köcher <[email protected]>

* Update test-utils/client/src/lib.rs

* fix

* deps fix

Co-authored-by: Bastian Köcher <[email protected]>