This commit allows separate running of Java and Ruby tests on Windows i.e. the same way as we currently do on unix (unit_tests.sh) via a cli argument.
If no argument has been supplied, both tests are run (as it does now).
The wrapper script is also rewritten from old batch style script to Powershell.
This work allows us to split the existing Windows CI job in a subsequent PR to separate steps, as we currently do on Linux.
Relates: https://github.com/elastic/logstash/issues/15566
