docs: clarify local tox setup and unit test matrix usage

Document tox 4 requirement and PATH fix for --user installs so make
targets use the correct tox binary. Describe how to use selective
unit test environments instead of running the full matrix.

Signed-off-by: Geetika Kapoor <gkapoor@redhat.com>

docs/developing.md

@@ -14,11 +14,23 @@
 
 ## Development environment
 
-To develop and to run tests you need to install `tox` and `tox-ansible` on
-your machine.
+Install `tox` (version 4 or newer) and `tox-ansible`:
 
 ```
-pip install --user tox tox-ansible
+pip install --user 'tox>=4' tox-ansible
+```
+
+With `--user`, `tox` is in `~/.local/bin`. Put it first on `PATH` before
+`make` targets that use `tox` (`build-venv`, `test-unit`, `test-sanity`):
+
+```
+export PATH="$HOME/.local/bin:$PATH"    # whole shell session
+```
+
+or
+
+```
+PATH="$HOME/.local/bin:$PATH" make <target>    # command only
 ```
 
 ### Virtualenv for development

docs/testing.md

@@ -27,6 +27,16 @@ Run unit tests with `tox-ansible` like so:
 make test-unit
 ```
 
+`make test-unit` runs the full matrix in parallel. For local development, run
+one environment instead. If you installed tox with `--user`, put `~/.local/bin`
+first on `PATH` before running `tox`. See [developing.md](developing.md).
+
+```
+tox -e unit-py3.11-2.16 --ansible --conf tox-ansible.ini
+```
+
+List available unit envs with `tox -f unit --ansible --conf tox-ansible.ini -l`.
+
 ### Running tests with ansible-test
 
 In order to test changes with `ansible-test`, it is recommended to bind mount
@@ -135,4 +145,3 @@ ansible-playbook -i examples/inventory.kubevirt.yml examples/play-delete.yml
 # terminate the environment
 hack/e2e-setup.sh --cleanup
 ```
-