This cuts out two tests by constructing test cases and iterating through
them, rather than having separate sets of tests for TCP and Unix listeners.
This is as per the feedback from #3121.

#2923 (review)

Adds tests for Unix sockets

* Adds a random suffix to temporary Unix sockets during testing

* Adds Unix domain socket tests for client (FAILING)

This adds Unix domain socket tests for the privval client. Right now,
one of the tests (TestRemoteSignerRetry) fails, probably because the
Unix domain socket state is known instantaneously on both sides by the
OS. Committing this to collaborate on the error.

* Removes extraneous logging

* Completes testing of Unix sockets client version

This completes the testing of the client connecting via Unix sockets.
There are two specific tests (TestSocketPVDeadline and
TestRemoteSignerRetryTCPOnly) that are only relevant to TCP connections.

* Renames test to show TCP-specificity

* Adds testing into closures for consistency (forgot previously)

* Moves test specific to RemoteSigner into own file

As per discussion on #3132, `TestRemoteSignerRetryTCPOnly` doesn't
really belong with the client tests. This moves it into its own file
related to the `RemoteSigner` class.

Release/v0.28.0