test coverage recording is flaky

[Samuel Allan]

stestr runs in parallel and coverage is only picking up coverage data from one process. I think the solution is to use --parallel-mode for coverage.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: main
Review: https://review.opendev.org/c/openstack/sunbeam-charms/+/906335

[Samuel Allan (samuelallan)]

I tested with coverage and we definitely need --parallel-mode. I opened the above patch to add it.

On current main, with 5 test runs of `tox -e cover`, the missing line counts vary:

```
31779
31813
31848
31864
31873
```

With the above patch, the missing line count is reduced and consistent between runs:

```
31405
31405
31405
31405
31405
```

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/openstack/sunbeam-charms/+/906335
Committed: https://opendev.org/openstack/sunbeam-charms/commit/caabd67022efb5b9f18e952c72cf9d3bb64b84b6
Submitter: "Zuul (22348)"
Branch: main

commit caabd67022efb5b9f18e952c72cf9d3bb64b84b6
Author: Samuel Allan <email address hidden>
Date: Tue Jan 23 15:35:28 2024 +1030

    Use --parallel-mode for test coverage

    stestr runs in parallel, which is good, but coverage needs to run
    in parallel awareness mode, so that data from all stestr processes
    is collected.

    Closes-Bug: #2050838

    Change-Id: I6a384f07d5063dc42b63943586d499f7647b1c64