Increase test coverage in the api package

[massenz]

This currently stands at ~30%, and we should bring it up to >70%:

└─( make cov

ok  	github.com/massenz/go-statemachine/api	  	0.016s	coverage: 30.9% of statements
ok  	github.com/massenz/go-statemachine/grpc	 	0.039s	coverage: 89.1% of statements
ok  	github.com/massenz/go-statemachine/pubsub	0.918s	coverage: 75.6% of statements
ok  	github.com/massenz/go-statemachine/server	0.014s	coverage: 72.4% of statements
ok  	github.com/massenz/go-statemachine/storage	0.013s	coverage: 72.3% of statements