Add more tests to miner/worker.go

[trianglesphere]

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Description

Add more tests. Ideas for some tests to add:

• Check that the pending feed and pending block are properly set
• Check that new blocks and the start channel interrupt the current creation and that the next one starts
• Check that the txs subscription properly feeds into the pending state.

Context

We only test two cases

1. That the system produces blocks with transactions (& that those transactions are included).
2. Test that the miner does not submit multiple blocks at the same height to istanbul

[mcortesi]

Some ideas on testing and worker spec

spec:

// Worker Spec

// PUBLIC METHODS:
// start()
// stop()
// isRunning()
// close()
// setValidator/TxFeeRecipient/Extra()
// pending() & pendingBlock()

// LISTENS TO:
// txPool NewTxsEvent()
// blockchain ChainHeadEvent()

// EMITS:
// type.Mux => NewMinedBlockEvent(block)
// event.Feed => pendingLogsFeed

// STATES
// - Stopped
// - Started
// - Closed

cases

// When call start()
//	If Stopped
//		isRunning() == True
//		should start working on a new block
//		should start mining blocks (check more than one)
//		pendingXXX() should return block currently being mined
//	If Started
//		isRunning() == True
//		should NOT interrupt current block construction
//		should keep mining blocks (check more than one)
//	If Closed
//		isRunning() == False
//		should FAIL
// When call stop()
//	If Stopped
//		should do nothing
//	If Started
//		should stop mining
//		should stop currently being mined block
//		pendingXXX() should return pending block obtained from txsEvent transactions
//	If Closed
//		should FAIL
// When call close()
//	If Stopped/Started
//		should stop listening to all subscriptions
//		should stop generating blocks
//	If Closed
//		should do nothing
// When new chainHead event
//	If Stopped
//		should interrupt currently being mined block
//		should start new pending block
//	If Started
//		should interrupt currently being mined block
//		should start new block and try to seal it
//	If Closed
//		should do nothing
// When new txsEvent
//	If Stopped
//		should include txs on currently pending block
//	If Started
//		should not block txsEvent emitter
//	If Closed
//		should do nothing
// When setXXX() is called
//	On Any state
//		should set XXX
//		XXX should only be used on "next block"
//		What happens with "pending block" since it's not constructed atomically?