[Tile-serving] [openstreetmap/mod_tile] Multithreading test fixes (#129)

[ǝɹʇʇɐʃǝ◖ xıʃǝɟ]

Thanks a lot for this fix! Reviewed, tested,[ squashed and merged it](https://github.com/openstreetmap/mod_tile/commit/1982516d491bf4f83c9756e4906fd900c5dff425) into `master`.

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