`pytest --rm success ...` not working as intended
**Summary**
`gmxapi.testsupport` adds support for a `--rm` option to pytest. The value of `success` is supposed to remove the testing temporary directory only when a test succeeds. At some point the logic seems to have broken, and the temporary directory is always removed unless `--rm never`.
**GROMACS version**
2020 and newer, but possibly dependent on `pytest` version.
**Steps to reproduce**
1. Install gmxapi, or build GROMACS with `-DGMX_PYTHON_PACKAGE=ON` and change to the `${CMAKE_BUILD_DIR}/python_packaging/src/gmxapi_staging` directory.
2. `python -m pytest <other pytest args> --rm success /path/to/python_packaging/src/test/<failing_test>`
**What is the current bug behavior?**
pytest `--rm never` works as expected, but `--rm success` proceeds as if `--rm always`
**What did you expect the correct behavior to be?**
As with `--rm never`, the temporary directory for a failed test should not be removed. Unless suppressed, a warning like the following would be visible:
```
===================================================================================== warnings summary ======================================================================================
test_subgraph.py::test_subgraph_simulation_extension
.../python_packaging/src/gmxapi_staging/gmxapi/testsupport.py:194: UserWarning: Temporary directory not removed: /<tmp_dir_path_prefix>/tmp6e9mml_d
```
**Possible fixes**
pytest provides `tmp_path` and `tmp_path_factory` fixtures that do most of what we need, but which have a different retention policy that avoids some of the other issues we've run into. We can build on these fixtures, instead.
Issues avoided:
* race conditions in directory removal
* custom policies on temporary directory retention
* extra custom command line argument processing
* appropriate reporting of the temporary directory path and status
issue