lnwallet: split off new TestForceCloseDustOutput case from TestForceClose
lnwallet: split off new TestForceCloseDustOutput case from TestForceClose

This commit extracts the ending dust adherence test case from the
existing TestForceClose test case into a distinct test case. With this
modification, we now ensure that the two new tests are focused and test
a single scenario at at time.

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