fatxpool: add tests for rejecting invalid transactions

[michalkucharczyk]

Unit tests for rejecting invalid transaction in different views configurations are required. Some improvements maybe required in ExternalWatcherContext::handle_invalidate_transaction.

[michalkucharczyk]

I reviewed and added some tests in #6661.

This issue should still be open, while there is a niche corner case that needs to be handled:

polkadot-sdk/substrate/client/transaction-pool/src/fork_aware_txpool/multi_view_listener.rs

Lines 248 to 254 in d62a90c

	//todo [#5477]
	// - handle corner case: this may happen when tx is invalid for mempool, but somehow
	// some view still sees it as ready/future. In that case we don't send the invalid
	// event, as transaction can still be included. Probably we should set some flag here
	// and allow for invalid sent from the view.
	// - add debug / metrics,
	false

This requires some rework: invalid transactions from view revalidation shall be sent to dropped_monitor_task and also force-removed from the view_store. After this we can safely send Invalid event.