Improve [test] template conditioning

[fcollonval]

Add the [test] dep group only when needed.
Add the [test] group to the pip install command when needed
Add the linking command to avoid confusion after only triggering pip install

[jtpio]

Or maybe the test optional dependencies array should not be defined if testing is not selected when generating the extension? (currently it defaults to an empty list)

Maybe it can be confusing to run pip install -e ".[test]" if they didn't choose to add tests.

[fcollonval]

Or maybe the test optional dependencies array should not be defined if testing is not selected when generating the extension? (currently it defaults to an empty list)

Maybe it can be confusing to run pip install -e ".[test]" if they didn't choose to add tests.

Done thanks for the review

[fcollonval]

@meeseeksdev please backport to 4.0