Add swift-testing support to the xctest runner

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Current LTS on :mac: macOS arm64curl -sS https://raw.githubusercontent.com/bazelbuild/continuous-integration/master/buildkite/bazelci.py?1741704591 -o bazelci.py && python3 bazelci.py runner --task=macos_latest
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Ran in 1m 43s
Last RC Bazel on :mac: macOS arm64curl -sS https://raw.githubusercontent.com/bazelbuild/continuous-integration/master/buildkite/bazelci.py?1741704591 -o bazelci.py && python3 bazelci.py runner --task=macos_last_rc
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Ran in 1m 23s
Last Green Bazel on :mac: macOS arm64curl -sS https://raw.githubusercontent.com/bazelbuild/continuous-integration/master/buildkite/bazelci.py?1741704591 -o bazelci.py && python3 bazelci.py runner --task=macos_last_green
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Total Job Run Time: 5m 52s