Add make gdk-test

What does this MR do and why?

This MR adds make gdk-test to run indexer in GDK mode for test environment.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Stop existing Zoekt test container

   gdk stop gitlab-zoekt-indexer-test

2. Set GDK_DIR

   export GDK_DIR=<PATH_TO_YOUR_GDK>

3. Start indexer in GDK test mode

   make gdk-test

4. Execute a zoekt spec

   bin/rspec ee/spec/requests/api/admin/search/zoekt_spec.rb