ipatests.beakerlib_plugin: Add argument of generated tests to test captions
    
    To differentiate between individual tests in BeakerLib output,
    the argument needs to be added to the test name. Since Nose
    doesn't provide a way to get the argument in a plugin,
    a `test_argument` attribute must be added to the test function
    to support this, simlarly to how `description` is used to set
    individual "docstrings".
    
    Add test_argument to the generated tests in the CA-less suite.