Create reusable test suite

[tzdybal]

Test suite should be created to establish some minimal acceptance testing framework for implementations.

First test should mimic actual life of a chain - calling all the methods in right order, repeating this for some number of blocks.

See also #16.

The API is very generic, so we need to think about other tests to add.

Example of reusable test suite is implemented in go-da.

[tzdybal]

To be able to create meaningful integration tests, a way to submit transactions is needed. This is implementation specific and is not a part of Execution API.

Proposed solution is to add TxInjector field to Suite so there is unified way to inject transactions in tests.

// TxInjector provides an interface for injecting transactions into a test suite.
type TxInjector interface {
	InjectTx(tx types.Tx)
}