stage: user test that tests adduser is called the right way

The issue with mocking subprocess.call() is that we will not
catch if arguments are passed as eg. integers. So instead use
the `mock_command()` helper so that a real binary is called.

stages/test/test_users.py

@@ -4,15 +4,18 @@ from unittest.mock import patch
 
 import pytest
 
-from osbuild.testutil import make_fake_tree
+from osbuild.testutil import make_fake_tree, mock_command
 
 STAGE_NAME = "org.osbuild.users"
 
-
-@pytest.mark.parametrize("user_opts,expected_args", [
+TEST_CASES = [
+    # user_opts,expected commandline args
     ({}, []),
-    ({"expiredate": 12345}, ["--expiredate", "12345"]),
-])
+    ({"expiredate": "12345"}, ["--expiredate", "12345"]),
+]
+
+
+@pytest.mark.parametrize("user_opts,expected_args", TEST_CASES)
 @patch("subprocess.run")
 def test_users_happy(mocked_run, tmp_path, stage_module, user_opts, expected_args):
     make_fake_tree(tmp_path, {
@@ -32,3 +35,22 @@ def test_users_happy(mocked_run, tmp_path, stage_module, user_opts, expected_arg
     args, kwargs = mocked_run.call_args_list[0]
     assert args[0] == ["chroot", tmp_path, "useradd"] + expected_args + ["foo"]
     assert kwargs.get("check")
+
+
+@pytest.mark.parametrize("user_opts,expected_args", TEST_CASES)
+def test_users_mock_bin(tmp_path, stage_module, user_opts, expected_args):
+    with mock_command("chroot", "") as mocked_chroot:
+        make_fake_tree(tmp_path, {
+            "/etc/passwd": "",
+        })
+
+        options = {
+            "users": {
+                "foo": {},
+            }
+        }
+        options["users"]["foo"].update(user_opts)
+
+        stage_module.main(tmp_path, options)
+        assert len(mocked_chroot.call_args_list) == 1
+        assert mocked_chroot.call_args_list[0][2:] == expected_args + ["foo"]