Fix coverage capture

- Move build/install into the test methods to reduce coupling with
  tox.ini
- Add the missing COVERAGE_PROCESS_START environment variable
- Move -p (parallel) to the config file
- When combining, pull coverage from the example directories because the
  package build process uses them as a working directory

.coveragerc

@@ -4,6 +4,7 @@ source_pkgs = incremental
 # List of directory names.
 source = tests
 branch = True
+parallel = True
 
 [paths]
 source =

tests/test_examples.py

@@ -5,15 +5,33 @@
 Tests for the packaging examples.
 """
 
-from twisted.trial.unittest import TestCase
+import os
 from importlib import metadata
+from subprocess import run
+
+from build import ProjectBuilder
+from twisted.python.filepath import FilePath
+from twisted.trial.unittest import TestCase
+
+
+TEST_DIR = FilePath(os.path.abspath(os.path.dirname(__file__)))
+
+
+def build_and_install(path):  # type: (FilePath) -> None
+    builder = ProjectBuilder(path.path)
+    pkgfile = builder.build("wheel", output_directory=os.environ["PIP_FIND_LINKS"])
+
+    # Force reinstall in case tox reused the venv.
+    run(["pip", "install", "--force-reinstall", pkgfile], check=True)
 
 
 class ExampleTests(TestCase):
     def test_setuppy_version(self):
         """
         example_setuppy has a version of 1.2.3.
         """
+        build_and_install(TEST_DIR.child("example_setuppy"))
+
         import example_setuppy
 
         self.assertEqual(example_setuppy.__version__.base(), "1.2.3")
@@ -23,6 +41,8 @@ def test_setuptools_version(self):
         """
         example_setuptools has a version of 2.3.4.
         """
+        build_and_install(TEST_DIR.child("example_setuptools"))
+
         import example_setuptools
 
         self.assertEqual(example_setuptools.__version__.base(), "2.3.4")

tox.ini

@@ -15,6 +15,7 @@ envlist =
 wheel = true
 wheel_build_env = build
 deps =
+    tests: build
     tests: coverage
     tests: coverage-p
     tests,mypy: twisted
@@ -27,6 +28,8 @@ extras =
 setenv =
     ; Suppress pointless chatter in the output.
     PIP_DISABLE_PIP_VERSION_CHECK=yes
+    tests: PIP_FIND_LINKS={distdir}
+    tests: COVERAGE_PROCESS_START={toxinidir}/.coveragerc
 
 commands =
     python -V
@@ -38,11 +41,9 @@ commands =
     tests: coverage --version
     tests: {envbindir}/trial --version
     tests: coverage erase
-    tests: coverage run -p {envbindir}/trial incremental
-    tests: coverage run -p -m pip install --use-pep517 --find-links {distdir} ./tests/example_setuppy
-    tests: coverage run -p -m pip -vvv install --use-pep517 --find-links {distdir} ./tests/example_setuptools
-    tests: coverage run -p {envbindir}/trial tests/test_examples.py
-    tests: coverage combine
+    tests: coverage run {envbindir}/trial incremental
+    tests: coverage run {envbindir}/trial tests/test_examples.py
+    tests: coverage combine . tests/example_setuppy tests/example_setuptools
     tests: coverage report
     tests: coverage html
     tests: coverage xml