GitHub Actions: Turn on merge queue (#108)

By default, when our PR check runs against each PR it is only testing
the specific commit HEAD of the PR branch, without necessarily also
considering the current HEAD of `main`, unless that commit has been
merged in manually into the PR branch. Merge queues provide a way of
automating this.

https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/configuring-pull-request-merges/managing-a-merge-queue

.github/workflows/tests.yml

@@ -1,6 +1,7 @@
 on:
-  - pull_request
-
+  pull_request:
+  merge_group:
+    types: [checks_requested]
 jobs:
   run-all-tests:
     runs-on: ubuntu-latest