test: fix expected empty stderr on cloud-init analyze boot

[blackboxsw]

Proposed Commit Message

test: fix expected empty stderr on cloud-init analyze boot

Fixed behavior in c9a520abef which no longer spits out container
on stderr. Update the integration tests to assert exit code and
empty stderr.

Additional Context

Failing integration tests on https://github.com/canonical/cloud-init/actions/runs/28896154678 and stonking.

Test Steps

CLOUD_INIT_OS_IMAGE=resolute CLOUD_INIT_CLOUD_INIT_SOURCE=IN_PLACE tox -e integration-tests -- tests/integration_tests/cmd/test_analyze.py::TestAnalyzeCommand::test_analyze_boot_ordered_timestamps

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)

[holmanb]

Thanks, @blackboxsw!