Add xctestrun based iOS unit test runner

[keith]

This test runner is a drop in replacement for the default iOS test runner that you can use by setting it on the runner attribute of an ios_unit_test. It uses the xctest CLI directly for logic tests, and uses xcodebuild test-without-building with a xctestrun file for hosted tests. It supports running tests in random order, and generating xcresult bundles from any type of test. It also supports running x86_64 test bundles on arm64 hosts. Examples can be found in the docs.