Add back in the pystog test data copy

I think this shouldn't have been removed from here https://github.com/mantidproject/mantid/pull/38404/files

Looks related to this nightly failure. https://builds.mantidproject.org/view/Nightly%20Pipelines/job/main_nightly_deployment/926/

This build is running the linux tests to make sure its fixed https://builds.mantidproject.org/job/build_packages_from_branch/949/

There is no associated issue.

To test:

Tests pass

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mantidproject / mantid