As part of fixing failing third-party tests, this PR significantly
rearchitects their specification and execution.
Third-party tests are now specified in a much more flexible format allowing
more customization on a per-package level. In addition, third-party tests are
now used to auto-generate a Github Actions workflow to perform the tests in
parallel. This not only gives faster feedback on PRs, but will also allow us
to more quickly narrow down on which packages are failing. An additional
workflow also confirms that local execution of third-party tests is consistent
with the Github Actions version. This workflow only runs when tests/thirdparty
itself is changed.
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