[tox] Split migractions tests from server environment These are the longest tests in this environment, so running them apart (in parallel of others) would hopefully speed up the whole suite.

Release notes
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.. toctree::
    :maxdepth: 1

    3.22
    3.21
    3.20
    3.19
    3.18
    3.17
    3.16
    3.15
    3.14