test: hydra_grouping tests race on a shared spec file #218

aqpbm-cli/tests/hydra_grouping.rs's spec() writes to hydra_grouping_<pid>_<dtype>.yaml. The tests run in parallel threads of one process, so the three i64 tests write the same file; one can read it mid-write and fail with missing field column_num (seen on #216's CI). Each call should write its own file.