Add xctrunner rule to create test bundles

[kapoorlakshya]

Adds new rule xctrunner which allows bundling of one or more xctests into a single XCTRunner.app. Primary use case is for running UI tests on real device farms, like BrowserStack, Sauce Labs, etc.

Usage

load("//apple:xctrunner.bzl", "xctrunner")

ios_ui_test(
    name = "HelloWorldSwiftUITests",
    minimum_os_version = "15.0",
    runner = "@build_bazel_rules_apple//apple/testing/default_runner:ios_xctestrun_ordered_runner",
    test_host = ":HelloWorldSwift",
    deps = [":UITests"],
)

xctrunner(
    name = "HelloWorldSwiftXCTRunner",
    test_targets = [":HelloWorldSwiftUITests"],
    testonly = True,
)

$ bazel build //examples/ios/HelloWorldSwift:HelloWorldSwiftXCTRunner

INFO: Analyzed target //examples/ios/HelloWorldSwift:HelloWorldSwiftXCTRunner (13 packages loaded, 2236 targets and 13 aspects configured).
INFO: Found 1 target...
Target //examples/ios/HelloWorldSwift:HelloWorldSwiftXCTRunner up-to-date:
  bazel-bin/examples/ios/HelloWorldSwift/HelloWorldSwiftXCTRunner.app
INFO: Elapsed time: 11.108s, Critical Path: 10.32s
INFO: 65 processes: 39 internal, 25 darwin-sandbox, 1 local.
INFO: Build completed successfully, 65 total actions

[brentleyjones]

Seems like AppleBundleInfo might be a better fit then, since we are producing a bundle?