func testPurchasingCoins_success() {
  self.app.launchArguments.append(contentsOf: [
    "-mockPurchase", "true"
  ])
  self.app.launch()
  // ... assert what should happen ...
}

func testPurchasingCoins_failure() {
  self.app.launchArguments.append(contentsOf: [
    "-mockPurchase", "false"
  ])
  self.app.launch()
  // ... assert what should happen ...
}