When importing from `pylib`, don't modify `sys.path` but use the fact
that both `test/` and `test/pylib/` directories contain an `__init__.py`
file, so `test.pylib` is a valid module if we start with `test/` as the
Python package root.
Both `pytest` and `mypy` (and I guess other tools) understand this
setup.
Also add an `__init__.py` to `test/topology/` so other modules under the
`test/` directory will be able to import stuff from `test/topology/`
(i.e. from `test.topology.X import Y`).
Closes#11467
