* Bump CI actions

Github retired macos 12 runners, so upgrade to more recent versions

* Exclude the MixedSwiftTestingXCTestSupport test from building on non-darwin platforms

* Specify the container to build documentation on

This partially solves the issue of swift testing's concurrent test runner tripping the 'nested polling expectation' error.
Solving this for synchronous polling expectations will be much more involved.
As synchronous polling expectations actually run all the test code on the main runloop.

Bumps [rexml](https://github.com/ruby/rexml) from 3.3.9 to 3.4.2.
- [Release notes](https://github.com/ruby/rexml/releases)
- [Changelog](https://github.com/ruby/rexml/blob/master/NEWS.md)
- [Commits](ruby/rexml@v3.3.9...v3.4.2)

---
updated-dependencies:
- dependency-name: rexml
  dependency-version: 3.4.2
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

* Revert "Remove duplicated runloop code (#1193)"

This reverts commit ae0648d.

* Re-remove duplicated runloop code

* Prevent failures on heavy load when doing continuous tests

* Add tests

* Fix typo

---------

Co-authored-by: Rachel Brindle <you@subluminal.net>