MNT Remove yarn test script

[emteknetnz]

Fixes broken yarn test

https://github.com/silverstripe/cwp-agencyextensions/runs/7380644680?check_suite_focus=true#step:12:228

Which is failing because it's trying to run cwp-agencyextensions/vendor/silverstripe/admin/thirdparty/jquery-changetracker/spec/unit/spec.js

While that's obviously incorrect, there's actually no reason to run yarn test at all as this module has no js tests in it

There's a condition in gha-run-tests that mean yarn test does not run if there is no 'test' script in package.json

https://github.com/silverstripe/gha-run-tests/blob/1/action.yml#L200

        if [[ $(cat package.json | jq -r '.scripts.test') != 'null' ]]; then
          echo "Running yarn test"
          yarn run test
        fi