miner.TestMiner is flakey

[piersy]

Test fails occasionally with panic, failed run.

This was the most recent commit on master d99a04a (the actual commit doesn't exist because I force pushed that branch, but I don't think the branch would have had an effect on this test)

=== RUN   TestMiner
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xa49f41]

goroutine 5 [running]:
github.com/celo-org/celo-blockchain/consensus/consensustest.(*MockEngine).Seal(0xc000332400, {0x10b87a0?, 0xc0003de000}, 0xc000096380)
	/runner/_work/celo-blockchain/celo-blockchain/consensus/consensustest/mockprotocol.go:384 +0x181
github.com/celo-org/celo-blockchain/miner.(*worker).submitTaskToEngine(0xc000228780, 0xc00003c400)
	/runner/_work/celo-blockchain/celo-blockchain/miner/worker.go:466 +0xb3
github.com/celo-org/celo-blockchain/miner.(*worker).constructAndSubmitNewBlock(0xc000228780, {0x10b7858, 0xc00003c080})
	/runner/_work/celo-blockchain/celo-blockchain/miner/worker.go:308 +0x515
github.com/celo-org/celo-blockchain/miner.(*worker).mainLoop.func1.1()
	/runner/_work/celo-blockchain/celo-blockchain/miner/worker.go:410 +0x3b
created by github.com/celo-org/celo-blockchain/miner.(*worker).mainLoop.func1
	/runner/_work/celo-blockchain/celo-blockchain/miner/worker.go:409 +0x1aa
FAIL	github.com/celo-org/celo-blockchain/miner	0.053s