Update federated_context_test to not depend on the native backend.

This backend has a TF dependency.

PiperOrigin-RevId: 678844674

tensorflow_federated/python/program/BUILD

@@ -112,7 +112,7 @@ py_test(
     srcs = ["federated_context_test.py"],
     deps = [
         ":federated_context",
-        "//tensorflow_federated/python/core/backends/native:execution_contexts",
+        "//tensorflow_federated/python/core/impl/context_stack:context_base",
         "//tensorflow_federated/python/core/impl/context_stack:context_stack_impl",
         "//tensorflow_federated/python/core/impl/types:computation_types",
         "//tensorflow_federated/python/core/impl/types:placements",

tensorflow_federated/python/program/federated_context_test.py

@@ -19,13 +19,19 @@
 from absl.testing import parameterized
 import numpy as np
 
-from tensorflow_federated.python.core.backends.native import execution_contexts
+from tensorflow_federated.python.core.impl.context_stack import context_base
 from tensorflow_federated.python.core.impl.context_stack import context_stack_impl
 from tensorflow_federated.python.core.impl.types import computation_types
 from tensorflow_federated.python.core.impl.types import placements
 from tensorflow_federated.python.program import federated_context
 
 
+class TestContext(context_base.SyncContext):
+
+  def invoke(self, comp, arg):
+    return None
+
+
 class ContainsOnlyServerPlacedDataTest(parameterized.TestCase):
 
   @parameterized.named_parameters(
@@ -149,17 +155,8 @@ def test_does_not_raise_value_error_with_context(self):
     with self.assertRaises(ValueError):
       federated_context.check_in_federated_context()
 
-  @parameterized.named_parameters(
-      (
-          'async_cpp',
-          execution_contexts.create_async_local_cpp_execution_context(),
-      ),
-      (
-          'sync_cpp',
-          execution_contexts.create_sync_local_cpp_execution_context(),
-      ),
-  )
-  def test_raises_value_error_with_context(self, context):
+  def test_raises_value_error_with_context(self):
+    context = TestContext()
     with self.assertRaises(ValueError):
       federated_context.check_in_federated_context()
 
@@ -183,7 +180,7 @@ def test_raises_value_error_with_context_nested(self):
       except TypeError:
         self.fail('Raised `ValueError` unexpectedly.')
 
-      context = execution_contexts.create_sync_local_cpp_execution_context()
+      context = TestContext()
       with context_stack_impl.context_stack.install(context):
         with self.assertRaises(ValueError):
           federated_context.check_in_federated_context()