test: Fix EventEmitter.MaxListenersExceededWarning warning (#132)
test: Fix EventEmitter.MaxListenersExceededWarning warning (#132)

When tests are run, node logs warnings - `MaxListenersExceededWarning: Possible EventEmitter memory
leak detected`. Fix warning by setting the max listeners to 20 on `process`(EventEmitter)
kentcdodds/cross-env