Run tests via meson

[jelly]

Currently the tests seem to require pip install . which I rather avoid on a machine. I tried running the tests by messing with PYTHONPATH but this is impossible as the C extension modules end up in build with an underscore and not all of them use a wrapper ie:

[jelle@carbon][~/projects/python-systemd]%PYTHONPATH=src:build/src/systemd pytest
===================================================================================== test session starts ======================================================================================
platform linux -- Python 3.13.7, pytest-8.4.2, pluggy-1.6.0
rootdir: /home/jelle/projects/python-systemd
configfile: pyproject.toml
plugins: cov-6.1.1, asyncio-0.26.0
asyncio: mode=Mode.STRICT, asyncio_default_fixture_loop_scope=None, asyncio_default_test_loop_scope=function
collected 2 items / 10 errors

============================================================================================ ERRORS ============================================================================================
____________________________________________________________________________ ERROR collecting src/systemd/daemon.py ____________________________________________________________________________
ImportError while importing test module '/home/jelle/projects/python-systemd/src/systemd/daemon.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/lib/python3.13/importlib/__init__.py:88: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
src/systemd/daemon.py:5: in <module>
    from ._daemon import (__version__,
E   ModuleNotFoundError: No module named 'systemd._daemon'

Doesn't work, so alternatively maybe the pip install can happen in a virtualenv but with system packages? Either way meson supports a custom test target which seems to be useful for this.