chore: add note on ui tests to contribution guide

[leonqadirie]

Not sure about the seemingly conflicting sentences:

All tests are located in libs/pavex_cli/tests and are launched using a custom test runner that you can find in libs/pavex_test_runner.

and

UI tests are located in libs/ui_tests and need to invoke pavexc.

Maybe we should reword the first one as 'All tests are collected via libs/pavex_cli/tests'?

Also happy to close, the hint would have saved me some time, though :)

[LukeMathWalker]

I clarified the setup in 934991b. Thanks for reporting it!