ci: configure tests to run on merge-queue

[romanc]

Description

This PR adds support for merging with merge queues. According to this guide it's a combination of adding the merge_group trigger and configuration in the admin interface. I can do the first part.

Using merge queues was talked about in the pace meeting in August. No concerns were raised. This PR supersedes #186.

How Has This Been Tested?

Will be tested with either this merge or the next one that goes in.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation: N/A
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules: N/A
• New check tests, if applicable, are included

[romanc]

@fmalatino would you (as repository admin) work with me to get merge queues going as a test in NDSL? Not sure if you can already configure it for this PR... If this PR has to merge first, we can merge it first, do the configuration and use PR #192 as a test-case to see if/that it works.