test/system: Add, use tcp_port_probe() to check for listeners rather than binds

_test_skopeo_credential_sharing() used port_is_free() to check if a
port has no active listeners. With the new implementation, this is
not equivalent anymore: a port might be in TIME_WAIT, so it's not
free, but the listener might be long gone.

Add tcp_port_probe() to check if there's an active listener on a
given port, and use it in _test_skopeo_credential_sharing().

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

test/system/150-login.bats

@@ -344,7 +344,7 @@ function _test_skopeo_credential_sharing() {
     fi
 
     # Make sure socket is closed
-    if ! port_is_free $PODMAN_LOGIN_REGISTRY_PORT; then
+    if tcp_port_probe $PODMAN_LOGIN_REGISTRY_PORT; then
         die "Socket still seems open"
     fi
 }

test/system/helpers.network.bash

@@ -286,3 +286,12 @@ function wait_for_port() {
 
     die "Timed out waiting for $host:$port"
 }
+
+# tcp_port_probe() - Check if a TCP port has an active listener
+# $1:	Port number
+# $2:	Optional address, 0.0.0.0 by default
+function tcp_port_probe() {
+    local address="${2:-0.0.0.0}"
+
+    : | nc "${address}" "${1}"
+}