rostest: Test node does not exist or is not executable

edit

I'm currently writing unit / integration tests for my nodes and i'm stuck adding them the the build scripts. When running

rostest my_pkg tests.test

I get the following error

[Testcase: testtest_vehicle_controller] ... FAILURE!
FAILURE: Test node [my_pkg/test_vehicle_controller.py] does not exist or is not executable
  File "/usr/lib/python2.7/unittest/case.py", line 329, in run
    testMethod()
  File "/opt/ros/lunar/lib/python2.7/dist-packages/rostest/runner.py", line 93, in fn
    self.fail(message)
  File "/usr/lib/python2.7/unittest/case.py", line 410, in fail
    raise self.failureException(msg)
--------------------------------------------------------------------------------

[ROSTEST]-----------------------------------------------------------------------

[testtest_vehicle_controller][failed]

SUMMARY
 * RESULT: FAIL
 * TESTS: 0
 * ERRORS: 0
 * FAILURES: 1

ERROR: The following tests failed to run:
 * testtest_vehicle_controller

my tests.test file looks like this

<launch>
  <test test-name="test_vehicle_controller" pkg="my_pkg" type="test_vehicle_controller.py" />
</launch>

my package structure looks like this

├── CMakeLists.txt
├── include
├── package.xml
├── src
│   ├── __init__.py
│   ├── __init__.pyc
│   ├── main
│   │   ├── __init__.py
│   │   └── vehicle_controller.py
│   └── tests
│       ├── context.py
│       ├── __init__.py
│       ├── Testing
│       │   └── Temporary
│       │       ├── CTestCostData.txt
│       │       └── LastTest.log
│       ├── tests.test
│       └── test_vehicle_controller.py
├── srv
│   └── ManipulatorPosition.srv
└── Testing
    └── Temporary
        ├── CTestCostData.txt
        └── LastTest.log

I don't see what I'm doing wrong here

edit:

my test_vehicle_controller.py looks like this

#!/usr/bin/env python
import unittest
PKG = 'my_pkg'
import roslib; roslib.load_manifest(PKG)

class AdvancedTestSuite(unittest.TestCase):
    """Advanced test cases."""

    def test_thoughts(self):
        self.assertIsNone(None)


if __name__ == '__main__':
    import rostest
    rostest.rosrun(PKG, 'test_vehicle_controller', AdvancedTestSuite)