Test PrepareRequest transactions verification

[AnnaShaleva]

#98 changes transactions routing structure and #87 adds block verification callback. We need to add unit-test that should check that transactions got from PrepareRequest are valid (pay special attention to the mempool validation rules, in-block transactions should obey all mempool validation rules). Specifically, in this unit-test we should ensure that transactions with invalid Nonce can't be included into proposed block and block with such malformed transactions is considered as invalid (i.e. WithVerifyBLock dBFT callback returns error for such malformed block).

Ref. #132 (review).

[AnnaShaleva]

Need to ensure that mempool tx validity check is included into block (and PrepareRequest) verification rules:

go-ethereum/core/txpool/validation.go

Lines 43 to 49 in 4d80ca7

	// ValidateTransaction is a helper method to check whether a transaction is valid
	// according to the consensus rules, but does not check state-dependent validation
	// (balance, nonce, etc).
	//
	// This check is public to allow different transaction pools to check the basic
	// rules without duplicating code and running the risk of missed updates.
	func ValidateTransaction(tx *types.Transaction, head *types.Header, signer types.Signer, opts *ValidationOptions) error {

[AnnaShaleva]

Depends on #314.