htlcswitch: use ticker.New instead of NewForce for test link

[Crypt-iQ]

NewForce's Pause method doesn't reset the ticker, so a test flake would occur in TestChannelLinkCancelFullCommitment where PendingCommitTicker.Pause() was called, but the underlying timer was still ticking. When PendingCommitTicker.Resume() was called, an unlucky Ticks() call could end up firing, leading to the link being shut down.

Running with:

go test -v -tags=dev,stdlog,debug -test.timeout=4m -run TestChannelLinkCancelFullCommitment

gave debug logs which helped narrow this down

Fixes https://github.com/lightningnetwork/lnd/issues/5437