feat: use self-signed jwt for service account (#665)

diff --git a/tests/test_credentials.py b/tests/test_credentials.py
index 0637b01..0633b38 100644
--- a/tests/test_credentials.py
+++ b/tests/test_credentials.py
@@ -142,16 +142,19 @@
 
 
 class RequiresScopedCredentialsImpl(credentials.Scoped, CredentialsImpl):
-    def __init__(self, scopes=None):
+    def __init__(self, scopes=None, default_scopes=None):
         super(RequiresScopedCredentialsImpl, self).__init__()
         self._scopes = scopes
+        self._default_scopes = default_scopes
 
     @property
     def requires_scopes(self):
         return not self.scopes
 
-    def with_scopes(self, scopes):
-        return RequiresScopedCredentialsImpl(scopes=scopes)
+    def with_scopes(self, scopes, default_scopes=None):
+        return RequiresScopedCredentialsImpl(
+            scopes=scopes, default_scopes=default_scopes
+        )
 
 
 def test_create_scoped_if_required_scoped():