Add Clearable test for OpenTracing propagation

Tests must be extended when the following PR is merged:
opentracing/opentracing-java#313

opentracing-span-propagation/src/main/java/nl/talsmasoftware/context/opentracing/SpanManager.java

@@ -44,7 +44,8 @@ public class SpanManager implements ContextManager<Span> {
      */
     @Override
     public Context<Span> getActiveContext() {
-        return new ManagedSpan(GlobalTracer.get().activeSpan(), null);
+        Span activeSpan = GlobalTracer.get().activeSpan();
+        return activeSpan == null ? null : new ManagedSpan(activeSpan, null);
     }
 
     /**

opentracing-span-propagation/src/test/java/nl/talsmasoftware/context/opentracing/SpanManagerTest.java

@@ -25,6 +25,7 @@
 import io.opentracing.util.ThreadLocalScopeManager;
 import nl.talsmasoftware.context.Context;
 import nl.talsmasoftware.context.ContextManager;
+import nl.talsmasoftware.context.ContextManagers;
 import nl.talsmasoftware.context.executors.ContextAwareExecutorService;
 import org.junit.After;
 import org.junit.Before;
@@ -215,4 +216,15 @@ public void testDeprecatedClassStillWorks() {
         newContext.close();
         newContext.close();
     }
+
+    @Test
+    public void testClearingAllContexts() {
+        Span span = mockTracer.buildSpan("test-span").start();
+        Scope scope = mockTracer.scopeManager().activate(span, false);
+        assertThat(new SpanManager().getActiveContext().getValue(), is(sameInstance(span)));
+
+        ContextManagers.clearActiveContexts();
+        // TODO Test after this is merged: https://github.com/opentracing/opentracing-java/pull/313
+//        assertThat(new SpanManager().getActiveContext(), is(nullValue()));
+    }
 }