Make with_c_api more robust and enable C API in most of saved_model_test.py.

This change makes the test_util.with_c_api decorator call
reset_default_graph() after enabling or disabling the C API instead of
creating a new Graph. This makes it more robust to tests that call
reset_default_graph(), which requires that the current default graph
isn't nested (which the C API-enabled Graph previously was).

In addition, enables the C API with saved_model_test.py (which
required the above change). A few tests still need further changes,
which I'll post in subsequent patches.

PiperOrigin-RevId: 183739148