Fix apparent race condition in test, observed in docker-machine on Mac (#459)

Fix apparent race condition in test, observed in docker-machine on Mac
Startup success detection appeared to be seeing container as having exited already
For the purposes of this test, all we need is a positive `isRunning` state, so
a container long running command (`top`) is sufficient to achieve this

Author: rnorth

core/src/test/java/org/testcontainers/junit/GenericContainerRuleTest.java

@@ -147,7 +147,7 @@ public static void setupContent() throws FileNotFoundException {
 
     @Test
     public void testIsRunning() {
-        try (GenericContainer container = new GenericContainer()) {
+        try (GenericContainer container = new GenericContainer().withCommand("top")) {
             assertFalse("Container is not started and not running", container.isRunning());
             container.start();
             assertTrue("Container is started and running", container.isRunning());