feat: add xctest support

[rillgen-saucelabs]

Description

Adds xctest test type support. Unlike xcuitest it doesn't require a "test" .ipa but a xctestrun file which is an Apple plist format as a descriptor. Here is what the configuration looks like:

apiVersion: v1alpha
kind: xctest
sauce:
  metadata:
    build: XCUITest Basic
  concurrency: 1
xctest:
  app: /Users/ricardoillgen/projects/saucelabs/rdc-xcuitest-test/apps/FlutterTestApp.ipa
  xcTestRunFile: /Users/ricardoillgen/projects/saucelabs/rdc-xcuitest-test/FlutterTest.xctestrun
suites:
  - name: "Basic XCTest"
    devices:
      - name: $DEVICE_NAME
        platformVersion: $PLATFORM_VERSION
artifacts:
  download:
    when: always
    match:
      - junit.xml
    directory: ./