Use docker ps to detect absence of docker permissions (jenkinsci#336)

The plugin BOM agents seem to have the `docker` command available but the
user running the agent is not authorized to use the `docker` command.
Previously that was detected by calling `docker ps` and detecting the
failure.

Restores a change made in

* jenkinsci#331

Testing done

Confirmed that I could see the same failure on a local computer as is seen on
https://ci.jenkins.io/job/Tools/job/bom/job/master/3968/testReport/org.jenkinsci.plugins.docker.workflow/DockerDSLTest/

The computer had Docker CE installed by the specific user running the
test did not have permission to access Docker.  Prior to this change,
the tests failed with the message:

CANNOT CONNECT TO THE DOCKER DAEMON AT UNIX:///VAR/RUN/DOCKER.SOCK. IS THE DOCKER DAEMON RUNNING?

After making this change, the tests pass on that computer with the
specific user that does not have permission to access Docker.

Author: MarkEWaite

src/test/java/org/jenkinsci/plugins/docker/workflow/DockerTestUtil.java

@@ -74,8 +74,14 @@ public static void assumeDocker(DockerOsMode osMode) throws Exception {
     public static void assumeDocker(DockerOsMode osMode, VersionNumber minimumVersion) throws Exception {
         Launcher.LocalLauncher localLauncher = new Launcher.LocalLauncher(StreamTaskListener.NULL);
         try {
-            ByteArrayOutputStream out = new ByteArrayOutputStream();
             int status = localLauncher
+                .launch()
+                .cmds(DockerTool.getExecutable(null, null, null, null), "ps")
+                .start()
+                .joinWithTimeout(DockerClient.CLIENT_TIMEOUT, TimeUnit.SECONDS, localLauncher.getListener());
+            Assume.assumeTrue("Docker working", status == 0);
+            ByteArrayOutputStream out = new ByteArrayOutputStream();
+            status = localLauncher
                 .launch()
                 .cmds(DockerTool.getExecutable(null, null, null, null), "version", "-f", "{{.Server.Os}}")
                 .stdout(out)