Cover tests https://nedbatchelder.com/blog/201908/dont_omit_tests_from_coverage.html

.appveyor.yml

@@ -77,7 +77,7 @@ test_script:
 - cd c:\pillow
 - '%PYTHON%\%PIP_DIR%\pip.exe install pytest pytest-cov'
 - c:\"Program Files (x86)"\"Windows Kits"\10\Debuggers\x86\gflags.exe /p /enable %PYTHON%\%EXECUTABLE%
-- '%PYTHON%\%EXECUTABLE% -m pytest -vx --cov PIL --cov-report term --cov-report xml Tests'
+- '%PYTHON%\%EXECUTABLE% -m pytest -vx --cov PIL --cov Tests --cov-report term --cov-report xml Tests'
 #- '%PYTHON%\%EXECUTABLE% test-installed.py -v -s %TEST_OPTIONS%' TODO TEST_OPTIONS with pytest?
 
 after_test:

.github/workflows/test-windows.yml

@@ -346,7 +346,7 @@ jobs:
         rem Add libraqm.dll (copied to INCLIB) to PATH.
         path %INCLIB%;%PATH%
         cd /D %GITHUB_WORKSPACE%
-        %PYTHON%\python.exe -m pytest -vx --cov PIL --cov-report term --cov-report xml Tests
+        %PYTHON%\python.exe -m pytest -vx --cov PIL --cov Tests --cov-report term --cov-report xml Tests
       shell: cmd
 
     - name: Upload errors

.travis/test.sh

@@ -2,7 +2,7 @@
 
 set -e
 
-python -m pytest -v -x --cov PIL --cov-report term Tests
+python -m pytest -v -x --cov PIL --cov Tests --cov-report term Tests
 
 # Docs
 if [ "$TRAVIS_PYTHON_VERSION" == "3.7" ]; then make doccheck; fi

Tests/README.rst

@@ -27,6 +27,6 @@ Run all the tests from the root of the Pillow source distribution::
 
 Or with coverage::
 
-    pytest --cov PIL --cov-report term
+    pytest --cov PIL --cov Tests --cov-report term
     coverage html
     open htmlcov/index.html