diff_drive_controller.test fails sometimes

[po1]

I built the diff drive controller after merging locally the following pull requests: #21, #22, #23, #24 and #25, build the tests, then ran them with make test. The first one failed with the following:

Running tests...
Test project /Users/paul/github/ros_controllers/diff_drive_controller/build
    Start 1: _ctest_diff_drive_controller_rostest_test_diff_drive_controller.test
1/4 Test #1: _ctest_diff_drive_controller_rostest_test_diff_drive_controller.test ..........***Failed   25.62 sec

Running make test again gave good results, all tests passed.