Test coverage

[james-pre]

This is the coverage Jest generates for v0.12.5:

• 51.94% Statements (1560/3003)
• 36.29% Branches (413/1138)
• 40.91% Functions (277/677)
• 52.07% Lines (1541/2959)

File		Statements		Branches		Functions		Lines
src	61.49%	428/696	46.92%	107/228	57.44%	108/188	61.88%	427/690
src/backends	4.87%	19/390	17.02%	16/94	7.07%	7/99	4.89%	19/388
src/backends/index	1.42%	1/70	0%	0/19	0%	0/17	1.42%	1/70
src/backends/port	68.75%	88/128	54.05%	20/37	50%	28/56	68.8%	86/125
src/backends/store	66.59%	295/443	46.46%	46/99	75%	51/68	66.43%	293/441
src/emulation	57.13%	729/1276	33.88%	224/661	33.33%	83/249	57.42%	715/1245

We should make sure to have a lot more coverage for 1.0