Make it possible to start two testing instances on same machine that are instantly connected

We have these interactive tests that run multiple briar desktop UIs with temporary accounts such as TestWithTwoConnectedTemporaryAccounts.kt. They add each other as contacts, too, however that takes quite a while (for some users at least, up to minutes) and makes interactive testing less efficient. I'd like to be able to make use of the internal managers and methods to establish a connection instantly.

One idea that came up was to use a special version of the LanPlugin that allows BRP (#373) however it might be easier and better maintainable to achieve this in a different manner.