How do I get the standalone test runner pex that would re-run the test goal that went into it creating it, so that an outsider could re-run failed integration tests without having the workspace from which the pants CLI was used? Does such an export option exist?

Not really, the test runner pex is an implementation detail. You could write a plugin to expose it, e.g., via

export

. But note that the test runner pex alone is not sufficient, since it won’t contain the code under test