Rewrites flakey test to not need I/O

This reduces load on cassandra test in attempts to help with travis.

See #1237

zipkin-storage/cassandra/src/test/java/zipkin/storage/cassandra/DeduplicatingExecutorTest.java

@@ -14,7 +14,7 @@
 package zipkin.storage.cassandra;
 
 import com.datastax.driver.core.BoundStatement;
-import com.datastax.driver.core.PreparedStatement;
+import com.datastax.driver.core.ResultSetFuture;
 import com.datastax.driver.core.Session;
 import com.google.common.collect.ImmutableSet;
 import com.google.common.reflect.Reflection;
@@ -35,6 +35,7 @@
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.failBecauseExceptionWasNotThrown;
 import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
 
 public class DeduplicatingExecutorTest {
   TestDeduplicatingExecutor executor = TestDeduplicatingExecutor.create(Futures::immediateFuture);
@@ -137,12 +138,14 @@ private void exceptionsArentCached() throws Exception {
   /**
    * This shows that any number of threads perform a computation only once.
    */
-  @Test(timeout = 2000L) // 1000 for the selects + expiration (which is 1 second)
+  @Test
   public void multithreaded() throws Exception {
-    Session session = CassandraTestGraph.INSTANCE.storage.get().session();
+    Session session = mock(Session.class);
     DeduplicatingExecutor executor =
         new DeduplicatingExecutor(session, TimeUnit.SECONDS.toMillis(1L));
-    PreparedStatement now = session.prepare("SELECT now() FROM system.local");
+    BoundStatement statement = mock(BoundStatement.class);
+    when(session.executeAsync(statement))
+        .thenAnswer(invocationOnMock -> mock(ResultSetFuture.class));
 
     int loopCount = 1000;
     CountDownLatch latch = new CountDownLatch(loopCount);
@@ -151,8 +154,8 @@ public void multithreaded() throws Exception {
     Collection<ListenableFuture<?>> futures = new ConcurrentLinkedDeque<>();
     for (int i = 0; i < loopCount; i++) {
       exec.execute(() -> {
-        futures.add(executor.maybeExecuteAsync(now.bind(), "foo"));
-        futures.add(executor.maybeExecuteAsync(now.bind(), "bar"));
+        futures.add(executor.maybeExecuteAsync(statement, "foo"));
+        futures.add(executor.maybeExecuteAsync(statement, "bar"));
         latch.countDown();
       });
     }
@@ -166,9 +169,9 @@ public void multithreaded() throws Exception {
     Thread.sleep(1000L);
 
     // Sanity check: we don't memoize after we should have expired.
-    assertThat(executor.maybeExecuteAsync(now.bind(), "foo"))
+    assertThat(executor.maybeExecuteAsync(statement, "foo"))
         .isNotIn(distinctFutures);
-    assertThat(executor.maybeExecuteAsync(now.bind(), "bar"))
+    assertThat(executor.maybeExecuteAsync(statement, "bar"))
         .isNotIn(distinctFutures);
   }