Provide way to use CLosure to assert tags values

dd-java-agent/testing/src/main/groovy/datadog/trace/agent/test/TagsAssert.groovy

@@ -51,6 +51,8 @@ class TagsAssert {
     def arg = args[0]
     if (arg instanceof Class) {
       assert ((Class) arg).isInstance(tags[name])
+    } else if (arg instanceof Closure) {
+      assert ((Closure) arg).call(tags[name])
     } else {
       assert tags[name] == arg
     }