synchrony_spec: Rewrite the second example

Not only it was broken (args undefined), which created failure in some
example that ran after it, it also didn't test what it said.

spec/synchrony_spec.rb

@@ -19,14 +19,23 @@
 
   describe "#next_tick" do
     it "should wrap next_tick into a Fiber context" do
-      Fiber.new {
-        df = EM::DefaultDeferrable.new
+      EM.synchrony {
+        begin
+          fired = false
+          f = Fiber.current
+
+          EM::Synchrony.next_tick do
+            fired = true
+            Fiber.current.should_not eq(f)
+          end
 
-        EM::Synchrony.next_tick do
-          df.succeed args
-          EM::Synchrony.sync(df).should == args
+          EM::Synchrony.interrupt
+
+          fired.should eq(true)
+        ensure
+          EM.stop
         end
-      }.resume
+      }
     end
   end