Code review comment for ~jawn-smith/autopkgtest-cloud:long-software-properties

> I think this should be OK, but can you confirm that making the test long lets
> it complete, and it's not hung?
>
> I'm seeing quite some variation in the history with test durations. That could
> be due to server load causing the tests to run slower sometimes, but I'd
> ideally like to know that it's not actually that there's a race condition
> which means we can hang forever - in which case making the tests 'long' would
> just hold up nodes for more time.
>
> https://autopkgtest.ubuntu.com/packages/software-properties/hirsute/armhf
>
> If you can't reproduce the timeout then the alternative strategy would be to
> merge this and someone agrees to watch the results for a bit to confirm they
> are consistently green now.

I've taken the following steps to demonstrate that adding this to long_tests will resolve the issue:

1) Run the test in a local armhf lxd instance 5 times with --timeout-test=999999999
    * All tests succeeded.
2) Run the test in a local armhf lxd instance with --timeout-test=10
    * Observed the same exact error as in the failed armhf autopkgtests.
3) Reviewed the source code to look for any possible race conditions
    * Found nothing. The test that is timing out simply iterates
      through all possible combinations of flags to 'add-apt-respository'.
    * This is a total of 638 combinations, which takes quite a while
    * None of these commands happen in parallel or asynchronously,
      so a race condition seems unlikely

In addition to these steps I'd be happy to watch the results for a bit to confirm they are consistently green now