Update outdated testing documentation (use pytest) (#38143)

* Update outdated testing documentation (use pytest)

The CONTRIBUTING.md's "How to submit pull requests" specified nosetests,
which is no longer used. (see: commit id f27dfbf, github pr #36201)
This commit updates the documentation to use pytest instead, and also
encourages the use of a virtual environment.

Config files are also updated to ignore the virtual environment.

.gitignore

@@ -1,3 +1,4 @@
 *~
 .DS_STORE
 *.pyc
+.venv/*

.yamllint

@@ -16,3 +16,6 @@ rules:
     allow-non-breakable-words: true
     ignore: |
       rosdep/*.yaml
+
+ignore: |
+  .venv

CONTRIBUTING.md

@@ -297,31 +297,27 @@ How to submit pull requests
 When submitting pull requests it is expected that they pass the unit tests for formatting.
 The unit tests enforce alphabetization of elements and a consistent formatting to keep merging as clean as possible.
 
-If you want to run the tests before submitting, first install the dependencies. Using `pip` is recommended.
+### Unit Testing
 
-```bash
-python3 -m pip install -r test/requirements.txt
-```
+It is recommended to use a virtual environment and pip to install the unit test dependencies.
+The test dependencies are listed in tests/requirements.txt.
 
-To run the tests run ``pytest -s test`` in the root of the repository.
-These tests require several dependencies that can be installed either from the ROS repositories or via pip (list built based on the content of [test/requirements.txt](https://github.com/ros/rosdistro/blob/master/test/requirements.txt):
-
-| Dependency   | Ubuntu package (<=22.04)| Pip package  |
-| :------------: | --------------------------------- | -------------- |
-| catkin_pkg     | python3-catkin-pkg                 | catkin-pkg     |
-| github         | python3-github                     | PyGithub       |
-| pytest         | python3-pytest                    | pytest         |
-| yaml           | python3-yaml                      | PyYAML         |
-| rosdep         | python3-rosdep                    | rosdep         |
-| rosdistro      | python3-rosdistro                  | rosdistro      |
-| ros_buildfarm  | python3-ros-buildfarm              | ros-buildfarm  |
-| unidiff        | python3-unidiff (Zesty and higher) | unidiff        |
-| yamllint       | yamllint                          | yamllint       |
-
-There is a tool [scripts/check_rosdep](./scripts/check_rosdep.py) which will check most formatting errors such as alphabetization and correct formatting.
-It is recommended to run it before submitting your contribution.
-
-For example, to check a change to `rosdep/base.yaml`:
 ```bash
-python3 scripts/check_rosdep.py rosdep/base.yaml
+# create the virtual environment
+python3 -m venv .venv
+
+# "activate" the virtual environment
+# this will let pip install dependencies into the virtual environment
+# use activate.zsh if you use zsh, activate.fish if you use fish, etc.
+source .venv/bin/activate
+
+# install the dependencies
+pip3 install -r test/requirements.txt
+
+# run the tests!
+pytest
 ```
+
+It is highly recommended to run the unit tests before submitting a pull request.
+(the CI system will run them anyways, but it will save you time)
+