force `subprocess.run` to check return code is 0

In general, this should help to capture unintentional errors.

Where the change has created failures, they have been fixed. (Mostly because the git repository that was looked for was missing.)

tests.py

@@ -32,8 +32,13 @@ def create_mock_git_repository(target_directory, create_directory=True):
         subprocess.run("echo 'fake content' > readme.txt", shell=True, check=True)
         subprocess.run("git add readme.txt", shell=True, check=True)
         subprocess.run("pwd", shell=True)
+
+        # if email and username are not available (such as on naked test container),
+        # git may complain. We set a temporary user for this one commit to work around
+        # that.
+        user_details = "-c user.name='Tes Ta' -c user.email='tester@some-ci.org'"
         subprocess.run(
-            'git commit -m "first commit" readme.txt', shell=True, check=True
+            f'git {user_details} commit -m "first commit" readme.txt', shell=True, check=True
         )