Add token_id unit test for impersonated service account credentials.

Author: nolanleastin

tests/oauth2/test_id_token.py

@@ -23,10 +23,16 @@
 from google.auth import transport
 from google.oauth2 import id_token
 from google.oauth2 import service_account
+from google.auth import impersonated_credentials
 
 SERVICE_ACCOUNT_FILE = os.path.join(
     os.path.dirname(__file__), "../data/service_account.json"
 )
+
+IMPERSONATED_SERVICE_ACCOUNT_FILE = os.path.join(
+    os.path.dirname(__file__), "../data/impersonated_service_account_authorized_user_source.json"
+)
+
 ID_TOKEN_AUDIENCE = "https://pubsub.googleapis.com"
 
 
@@ -262,6 +268,15 @@ def test_fetch_id_token_credentials_from_explicit_cred_json_file(monkeypatch):
     assert cred._target_audience == ID_TOKEN_AUDIENCE
 
 
+def test_fetch_id_token_credentials_from_impersonated_cred_json_file(monkeypatch):
+    ## Test: Can I Fetch ID token Credentials?
+    monkeypatch.setenv(environment_vars.CREDENTIALS, IMPERSONATED_SERVICE_ACCOUNT_FILE)
+
+    cred = id_token.fetch_id_token_credentials(ID_TOKEN_AUDIENCE)
+    assert isinstance(cred, impersonated_credentials.IDTokenCredentials)
+    assert cred._target_audience == ID_TOKEN_AUDIENCE
+
+
 def test_fetch_id_token_credentials_no_cred_exists(monkeypatch):
     monkeypatch.delenv(environment_vars.CREDENTIALS, raising=False)