Use LastStateTransitionTime to populate last_flapped for get_interfaces() in IOS-XR

[jlixfeld]

Currently, last_flapped for get_interfaces() is hard coded to -1.

In at least IOS-XR 7.1.2, Last link flapped from show interface is exposed by <Get><Operational><Interfaces></Interfaces></Operational></Get> as LastStateTransitionTime.

This can be used to populate last_flapped, with -1 being a default for backwards compatibility where LastStateTransitionTime is not exposed.

[jlixfeld]

While trying to implement the required tests, I'm hung up here:

./test/iosxr/test_getters.py::TestGetter::test_get_interfaces[normal] Failed: [undefined]AssertionError
self = <test.iosxr.test_getters.TestGetter object at 0x7f4c738e6ce0>
test_case = 'normal'

    @wrap_test_cases
    def test_get_interfaces(self, test_case):
        """Test get_interfaces."""
        get_interfaces = self.device.get_interfaces()
        assert len(get_interfaces) > 0
    
        for interface, interface_data in get_interfaces.items():
>           assert helpers.test_model(models.InterfaceDict, interface_data)
E           AssertionError

napalm/base/test/getters.py:165: AssertionError