Vary number of ranks in CI tests

Sometimes, tests fail only when run with a certain number of ranks, so we should probably run CI tests with different numbers of ranks (if that is possible in GitHub actions).

Start by locating the repository's GitHub Actions CI configuration and the CI tests that launch multiple ranks. Define the rank counts to exercise and verify the workflow runs the tests at each count; done means varied-rank failures are reported by CI.

Written by the indexing model from the issue text.