[enhancement]: system-dependent unit tests

# Enhancement

While many improvements have been made to make unit tests more independent of the runtime environment, there is more work to be done. A low-effort smoke test for unintentional system dependencies is to run them on a non-Linux Unix. GH Actions makes it very easy to test on macos. Using the following diff:

```diff
commit 33dba15d07448c714b57e5eacebd54907a57cea1 (HEAD -> main)
Author: Brett Holman
Date: Mon May 27 15:33:59 2024 -0600

run on macos

diff --git a/.github/workflows/unit.yml b/.github/workflows/unit.yml
index ec054f86c..67472b1f2 100644
--- a/.github/workflows/unit.yml
+++ b/.github/workflows/unit.yml
@@ -14,24 +14,19 @@ jobs:
unittests:
strategy:
matrix:
- python-version: ["3.7", "3.8", "3.9", "3.10", "3.11", "3.12"]
+ python-version: ["3.10", "3.11", "3.12"]
toxenv: [py3]
experimental: [false]
check-latest: [false]
continue-on-error: [false]
include:
- - python-version: "3.6"
- toxenv: lowest-supported
- continue-on-error: false
- check-latest: false
- experimental: false
- python-version: "3.13-dev"
toxenv: py3
check-latest: true
experimental: true
continue-on-error: true
name: unittest / ${{ matrix.toxenv }} / python ${{matrix.python-version}}
- runs-on: ubuntu-20.04
+ runs-on: macos-latest
continue-on-error: ${{ matrix.experimental }}
steps:
- name: "Checkout"
```

I see 37 failures:
```
= 37 failed, 5217 passed, 11 skipped, 1 deselected, 10 xfailed, 315 warnings in 62.78s (0:01:02) =
py3: exit 1 (64.80 seconds) /Users/runner/work/cloud-init/cloud-init> .tox/py3/bin/python -m pytest -vvvv --showlocals --durations 10 -m 'not hypothesis_slow' --cov=cloudinit --cov-branch tests/unittests pid=4525
py3: FAIL code 1 (68.78=setup[3.98]+cmd[64.80] seconds)
evaluation failed :( (68.82 seconds)
```

See the full log:

[macos-unit-test-failures.log](https://github.com/canonical/cloud-init/files/15462158/macos-unit-test-failures.log)

Most of these failures are the result of assuming temporary file locations, filepath names, subcommand differences, and a couple are just due to unchangeable things (ex: AF_SOCKET not existing on macos) - all trivial things to fix. While cloud-init has no intention of running on macos, a Unix environment such as macos is a cheap way to get coverage of these tests, and would allow developers on macos to more easily contribute to this project.