As part of the work done in #1979686 the unit test requirements file was moved from tests/requirements.txt to tests/unit/requirements.txt, however this change is not reflected in the Makefile target ending up with not being able to run `git commit` during development.
miles@t440:~/tmp/juju-lint$ git branch -vv
* master 03479fa [origin/master] unit tests to check if rules file on contrib folder load correctly.
miles@t440:~/tmp/juju-lint$ make dev-environment
Creating virtualenv and installing pre-commit
created virtual environment CPython3.8.10.final.0-64 in 301ms
creator CPython3Posix(dest=/home/miles/tmp/juju-lint/.venv, clear=False, global=False)
seeder FromAppData(download=False, pip=latest, setuptools=latest, wheel=latest, pkg_resources=latest, via=copy, app_data_dir=/home/miles/.local/share/virtualenv/seed-app-data/v1.0.1.debian.1)
activators BashActivator,CShellActivator,FishActivator,PowerShellActivator,PythonActivator,XonshActivator
ERROR: Could not open requirements file: [Errno 2] No such file or directory: 'tests/requirements.txt'
make: *** [Makefile:49: dev-environment] Error 1
