htlcswitch/link_test: adds TestChannelLinkShutdownDuringForward
htlcswitch/link_test: adds TestChannelLinkShutdownDuringForward

This commit adds a test that verifies Stop does not block
if the link is concurrently forwarding incoming Adds to
the switch. This test fails prior to the commits that
thread through the link's quit channel.

aakselrod/lnd,LightningNetwork/lnd,aakselrod/lnd,LightningNetwork/lnd