Run tests using previously created simulator if possible

This only applies if there is no destination configured on '.batler.yml'.
If the stored simulator does not exist, a new one is created.
If there is no simulator stored in cache, a new one is created.

simctl/simulator.go

@@ -2,14 +2,22 @@ package simctl
 
 import (
 	"fmt"
+	"io/ioutil"
 	"os"
 	"os/exec"
 	"strings"
 )
 
+const simulatorCacheFilename = "/tmp/batler-simulator"
+
 // CreateSimulator creates an arbitrary iOS simulator from existing runtimes and devicetypes.
 // This method returns the identifier for the newly created simulator or an error, otherwise.
 func CreateSimulator(xcodePath string) (string, error) {
+	cachedSimulatorID, err := getSimulatorFromCache(xcodePath)
+	if err == nil {
+		return cachedSimulatorID, nil
+	}
+
 	runtimes, err := ListRuntimes(xcodePath)
 	if err != nil {
 		return "", fmt.Errorf("could not get runtimes: %w", err)
@@ -34,13 +42,50 @@ func CreateSimulator(xcodePath string) (string, error) {
 			}
 
 			simulatorID := strings.TrimRight(string(output), "\r\n")
+
+			if err := saveSimulatorOnCache(simulatorID); err != nil {
+				// TODO (vitor.araujo): replace fmt by logging
+				fmt.Printf("failed to cache simulator id: %v\n", err)
+			}
+
 			return simulatorID, nil
 		}
 	}
 
 	return "", fmt.Errorf("could not create simulator from device types and runtimes")
 }
 
+func saveSimulatorOnCache(simulatorID string) error {
+	return ioutil.WriteFile(simulatorCacheFilename, []byte(simulatorID), 0600)
+}
+
+func getSimulatorFromCache(xcodePath string) (string, error) {
+	output, err := ioutil.ReadFile(simulatorCacheFilename)
+	if err != nil {
+		return "", fmt.Errorf("could not read cache file: %w", err)
+	}
+
+	simulatorID := strings.TrimRight(string(output), "\r\n")
+	if !simulatorExists(simulatorID, xcodePath) {
+		return "", fmt.Errorf("cached simulator with ID=%s does not exist", simulatorID)
+	}
+
+	return simulatorID, nil
+}
+
+func simulatorExists(deviceID, xcodePath string) bool {
+	cmd := exec.Command("xcrun", "simctl", "list", "devices", deviceID)
+	cmd.Env = os.Environ()
+	cmd.Env = append(cmd.Env, fmt.Sprintf("DEVELOPER_DIR=%s", xcodePath))
+
+	output, err := cmd.Output()
+	if err != nil {
+		return false
+	}
+
+	return strings.Contains(string(output), deviceID)
+}
+
 func createDeviceName(runtimeID, deviceTypeID string) string {
 	runtimeIDParts := strings.Split(runtimeID, ".")
 	osVersion := runtimeIDParts[len(runtimeIDParts)-1]