Shutdown default context after tests (#95)

This fixes some fatal runtime errors during testing.

Signed-off-by: Jacob Perron <jacob@openrobotics.org>

Author: jacobperron

rcljava/src/test/java/org/ros2/rcljava/publisher/PublisherTest.java

@@ -33,5 +33,6 @@ public final void testCreate() {
     assertEquals(node.getHandle(), publisher.getNodeReference().get().getHandle());
     assertNotEquals(0, publisher.getNodeReference().get().getHandle());
     assertNotEquals(0, publisher.getHandle());
+    RCLJava.shutdown();
   }
 }

rcljava/src/test/java/org/ros2/rcljava/subscription/SubscriptionTest.java

@@ -36,5 +36,6 @@ public void accept(final std_msgs.msg.String msg) {}
     assertEquals(node.getHandle(), subscription.getNodeReference().get().getHandle());
     assertNotEquals(0, subscription.getNodeReference().get().getHandle());
     assertNotEquals(0, subscription.getHandle());
+    RCLJava.shutdown();
   }
 }

rcljava/src/test/java/org/ros2/rcljava/timer/TimerTest.java

@@ -79,5 +79,6 @@ public final void testCreate() {
     assertTrue(timer.isCanceled());
 
     assertEquals(4, timerCallback.getCounter());
+    RCLJava.shutdown();
   }
 }