How can I use test nodes in rostest which are not part of rostest itself?

edit

I want to test a ROS node with rostest but with a test node which is not included in rostest itself. Means instead of using the hztest test node like in this rostest test...

<launch>
  <node name="talker" pkg="rospy" type="talker.py" />

  <param name="hztest1/topic" value="chatter" />  
  <param name="hztest1/hz" value="10.0" />
  <param name="hztest1/hzerror" value="0.5" />
  <param name="hztest1/test_duration" value="5.0" />    
  <param name="hztest1/wait_time" value="21.0" />    
  <test test-name="hztest_test" pkg="rostest" type="hztest" name="hztest1" />
</launch>

... I would like to use my own test node owntest from my own ROS Python package ownpackage instead:

<launch>
  <node name="talker" pkg="rospy" type="talker.py" />

  <param name="owntest1/topic" value="chatter" />  
  ...
  <param name="owntest1/test_duration" value="5.0" />    
  <test test-name="owntest_test" pkg="ownpackage" type="owntest" name="owntest1" />
</launch>

I built and installed ownpackage with catkin_make --pkg ownpackage, cd /build/ownpackage and make install. However if I run the test I get an error because rostest does not know about the existence of owntest:

FAILURE: Test Fixture Nodes ['owntest'] failed to launch
  File "/usr/lib/python2.7/unittest/case.py", line 329, in run
    testMethod()
  File "/home/florian/ws_catkin/src/ros_comm/tools/rostest/src/rostest/runner.py", line 121, in fn
    self.assert_(not failed, "Test Fixture Nodes %s failed to launch"%failed)
  File "/usr/lib/python2.7/unittest/case.py", line 422, in assertTrue
    raise self.failureException(msg)

How can I let rostest know about the existence of the owntest node from ownpackage that rostest can use it in a test?

Or alternativelly: What is the best approach/development procedure when I would place owntest into ros_comm/rostest/nodes instead of into ownpackage?

Edit: If I start the roscore and try to run a rostest test node with rosrun rostest hztest I got the following output:

[ROSUNIT] Outputting test results to /home/kromer/.ros/test_results/rostest/rosunit-hztest.xml
test_hz ... FAILURE!
FAILURE: hztest not initialized properly. Parameter ['~hz'] not set. debug[/hztest] debug[/hztest/hz]
  File "/usr/lib/python2.7/unittest/case.py", line 331, in run
    testMethod()
  File "/opt/ros/indigo/share/rostest/nodes/hztest/hztest", line 89, in test_hz
    self.fail('hztest not initialized properly. Parameter [%s] not set. debug[%s] debug[%s]'%(str(e), rospy.get_caller_id(), rospy.resolve_name(e.args[0])))
  File "/usr/lib/python2.7/unittest/case.py", line 412, in fail
    raise self.failureException(msg)
--------------------------------------------------------------------------------
-------------------------------------------------------------
SUMMARY:
 * RESULT: FAIL
 * TESTS: 1
 * ERRORS: 0 []
 * FAILURES: 1 [test_hz]

I would expect the same for my package. I diff the files of my package and rostest to figure out what I missed...