testsuite: Supress stderr when running integration tests

[mpickering]

This fixes a problem where test output was leaking into GitHub Actions results (#8419), making the output unnecessarily verbose and harder to read.

This makes the test output much cleaner while still preserving all the diagnostic information when tests fail.

Fixes #8419

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[ulysses4ever]

No updates for two weeks: just a gentle ping @mpickering! I know it's low priority but it'd be nice to get it done, and it's really close now. As usual, feel free to signal that you'd like someone else to pick it up.

@Bodigrim do you want to resolve your comments that were addressed?

[Bodigrim]

Sure, I resolved comments which were addressed. The remaining two are suggestions, but not blockers, up to you.

[mpickering]

I have resolved @Bodigrim's comments now.

[ulysses4ever]

@Bodigrim do you think you could give a formal GitHub approval here?

[mergify]

This pull request has been removed from the queue for the following reason: checks failed.

The merge conditions cannot be satisfied due to failing checks:

You may have to fix your CI before adding the pull request to the queue again.
If you update this pull request, to fix the CI, it will automatically be requeued once the queue conditions match again.
If you think this was a flaky issue instead, you can requeue the pull request, without updating it, by posting a @mergifyio requeue comment.

[ulysses4ever]

@mergify rebase

[mergify]

rebase

☑️ Nothing to do

• any of:
  • #commits > 1 [📌 rebase requirement]
  • #commits-behind > 0 [📌 rebase requirement]
  • -linear-history [📌 rebase requirement]
• -closed [📌 rebase requirement]
• -conflict [📌 rebase requirement]
• queue-position = -1 [📌 rebase requirement]

[ulysses4ever]

@mergify requeue

[mergify]

requeue

✅ The queue state of this pull request has been cleaned. It can be re-embarked automatically

[ulysses4ever]

Seemed like a transient Windows failure, so I tried to restart.