add test for mismatched domain

tests/commands/conftest.py

@@ -1,6 +1,8 @@
 from pytest import fixture
 from typer.testing import CliRunner
 
+from data_safe_haven.administration.users.entra_users import EntraUsers
+from data_safe_haven.administration.users.research_user import ResearchUser
 from data_safe_haven.config import (
     Context,
     ContextManager,
@@ -260,3 +262,14 @@ def tmp_contexts_none(tmp_path, context_yaml):
     with open(config_file_path, "w") as f:
         f.write(context_yaml)
     return tmp_path
+
+
+@fixture
+def mock_entra_user_list(mocker):
+    test_user = ResearchUser(
+        given_name="Harry",
+        surname="Lime",
+        sam_account_name="harry.lime",
+        user_principal_name="[email protected]",
+    )
+    mocker.patch.object(EntraUsers, "list", return_value=[test_user])

tests/commands/test_users.py

@@ -52,6 +52,26 @@ def test_invalid_shm(
         assert result.exit_code == 1
         assert "Have you deployed the SHM?" in result.stdout
 
+    def test_mismatched_domain(
+        self,
+        mock_graphapi_get_credential,  # noqa: ARG002
+        mock_pulumi_config_no_key_from_remote,  # noqa: ARG002
+        mock_shm_config_from_remote,  # noqa: ARG002
+        mock_sre_config_from_remote,  # noqa: ARG002
+        mock_entra_user_list,  # noqa: ARG002
+        runner,
+        tmp_contexts,  # noqa: ARG002
+    ):
+        result = runner.invoke(
+            users_command_group, ["register", "-u", "harry.lime", "sandbox"]
+        )
+
+        assert result.exit_code == 0
+        assert (
+            "principal domain name must match the domain of the SRE to be registered"
+            in result.stdout
+        )
+
     def test_invalid_sre(
         self,
         mock_pulumi_config_from_remote,  # noqa: ARG002