Run pytest under coverage.py

This should still run the same pytest tests as usual, but will now
measure code coverage for anything under `tests/` and will fail if not
all code paths are exercised.

Would have prevented #122

Consider expanding coverage to the actual fauxmo code (`src/`)
eventually, but this should at least help ensure that the tests I
*think* are running are actually running.

pyproject.toml

@@ -54,6 +54,7 @@ fauxmo = "fauxmo.cli:cli"
 [project.optional-dependencies]
 test = [
     "black==23.1.0",
+    "coverage==7.4.1",
     "flake8-docstrings==1.7.0",
     "flake8-import-order==0.18.2",
     "flake8==6.0.0",

tox.ini

@@ -5,7 +5,8 @@ isolated_build = True
 [testenv]
 extras = test
 commands =
-    python -m pytest {posargs:--verbose --showlocals} tests/
+    python -m coverage run --source=tests -m pytest {posargs:--verbose --showlocals} tests/
+    python -m coverage report --fail-under=100
 
 [testenv:lint]
 extras = test