Expanding tests to cover Unix sockets version of client (#3132)

* 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.

privval/socket_test.go

@@ -29,7 +29,7 @@ type listenerTestCase struct {
 // testUnixAddr will attempt to obtain a platform-independent temporary file
 // name for a Unix socket
 func testUnixAddr() (string, error) {
-	f, err := ioutil.TempFile("", "tendermint-privval-test")
+	f, err := ioutil.TempFile("", "tendermint-privval-test-*")
 	if err != nil {
 		return "", err
 	}