Revalidate transactions only on latest best block (#6824)

* Revalidate transactions only on latest best block

We should revalidate transactions only on the latest best block and not
on any arbitrary block. The revalidation before failed when there were
multiple blocks on the height given to the revalidation function, but no
block was imported as best block.

* Update test-utils/runtime/transaction-pool/src/lib.rs

Co-authored-by: Jaco Greeff <[email protected]>

* Fix tests

* Only process best blocks in the transaction pool

Co-authored-by: Jaco Greeff <[email protected]>