[ci] Box all tests in pytest-forked

[driazati]

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This wraps all tests in pytest-forked (with n=1) so that segfaults and other process-terminations are properly reported by pytest. With this reporting on issues like #12311 should be much better (segfaults become like any other test failure)

cc @Mousius @areusch @gigiblender

[github-actions[bot]]

Built docs for commit fcf46aee59aac0f10005f0561236666f368dd28e can be found here.