This patch improves _get_ipv4_forward_table by doing only
one call to iphlpapi.GetIpForwardTable. Previously two calls were
made, where the second call should have been called only when
the first one failed with INSUFFICIENT_BUFFER. This patch also
adds tests for _get_ipv4_forward_table, which was previously
untested.
Change-Id: I5728948557e749d0e0005a9d90c12e4093dc3307