Update SessionTest.testFeedShapeCompatibility to work with C API enabled.

This test got lost in the transition. Prior to enabling the C API,
some constant node whose values were used for shape inference would be
marked as unfeedable in tensor_util.constant_value
(https://github.com/tensorflow/tensorflow/blob/r1.8/tensorflow/python/framework/tensor_util.py#L810).

This shape inference path is no longer used with the C API enabled, so
the constant node is successfully fed, triggering a runtime shape error.

This is arguably a regression, but given that the Python code wouldn't
mark all nodes evaluated during shape inference as unfeedable, it
seems ok to relax the check a little more.

PiperOrigin-RevId: 197002741