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 +32,15 @@ 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,
         )
 
 
@@ -82,17 +89,18 @@ def test_prepare_environment(tmp_path):
     assert release_base_dir.exists()
     # check for spack-environments directory
     assert spack_environments in os.listdir(release_base_dir)
-    # check if the git branch is correctly checked out
-    assert (
-        subprocess.run(
-            f"cd {str(release_base_dir/spack_environments)} && git branch",
-            shell=True,
-            capture_output=True,
-        )
-        .stdout.decode("utf-8")
-        .split("\n")[0]
-        == f"* {mpsd_release_to_test}"
+
+    # check if the git branch is correctly checked out. We expect output such as
+    # git_branch_stdout = '* dev-23a\n  develop\n'
+    # The entry with the '* ' prefix is the active branch.
+    git_branch_output_raw = subprocess.run(
+        f"cd {str(release_base_dir/spack_environments)} && git branch",
+        shell=True,
+        capture_output=True,
     )
+    git_branch_stdout = git_branch_output_raw.stdout.decode("utf-8")
+    assert f"* {mpsd_release_to_test}" in git_branch_stdout
+
     # check that result is a list and contains atleast ['global','foss2021a-mpi']
     assert isinstance(result, list)
     assert "global" in result