multi: fix unit test for high CPU load envs

It seems that the GitHub CI runners share a CPU and therefore only have
very few ressources available. This causes the wallet unlocker and breach
arbiter tests to fail if we don't give them enough time to complete.

docs/release-notes/release-notes-0.14.0.md

@@ -84,6 +84,11 @@ you.
 
 * [The `lnwire` fuzz tests have been fixed and now run without crashing.](https://github.com/lightningnetwork/lnd/pull/5395)
 
+* [A flake in the race unit
+  tests](https://github.com/lightningnetwork/lnd/pull/5659) was addressed that
+  lead to failed tests sometimes when the CPU of the GitHub CI runner was
+  strained too much.
+
 ## Documentation
 
 * [Outdated warning about unsupported pruning was replaced with clarification that LND **does**