* Consolidates deadline tests for privval Unix/TCP
Following on from #3115 and #3132, this converts fundamental timeout
errors from the client's `acceptConnection()` method so that these can
be detected by the test for the TCP connection.
Timeout deadlines are now tested for both TCP and Unix domain socket
connections.
There is also no need for the additional secret connection code: the
connection will time out at the `acceptConnection()` phase for TCP
connections, and will time out when attempting to obtain the
`RemoteSigner`'s public key for Unix domain socket connections.
* Removes extraneous logging
* Adds IsConnTimeout helper function
This commit adds a helper function to detect whether an error is either
a fundamental networking timeout error, or an `ErrConnTimeout` error
specific to the `RemoteSigner` class.
* Adds a test for the IsConnTimeout() helper function
* Separates tests logically for IsConnTimeout