channeldb/graph_test: ensure policies for an edge have different timestamps
channeldb/graph_test: ensure policies for an edge have different
timestamps

In this commit, we ensure policies for edges we create in
TestChanUpdatesInHorizon have different update timestamps. This ensures
that there are two entries per edge in the edge update index. Because of
this, the test will fail because ChanUpdatesInHorizon will return
duplicate channel edges due to looking at all the entries within the
edge update index. This will be addressed in a future commit to allow
the set of tests to pass once again.

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