[ros2] Use pytest instead of nose

[sloretz]

Should fix foxy dev job: http://build.ros2.org/job/Fdev__resource_retriever__ubuntu_focal_amd64/1/testReport/junit/(root)/resource_retriever/test_test/

ImportError while importing test module '/tmp/ws/src/resource_retriever/resource_retriever/test/test.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
../../src/resource_retriever/resource_retriever/test/test.py:31: in <module>
    from nose.tools import raises
E   ModuleNotFoundError: No module named 'nose'