Reproduce bug in qrexec tests

[DemiMarie]

The qrexec tests are broken and fail intermittently. This patch makes the bug obvious: by adding a sleep, it causes the test to fail right away, reliably.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.72%. Comparing base (5de215d) to head (87f187d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #661      +/-   ##
==========================================
- Coverage   69.79%   69.72%   -0.07%     
==========================================
  Files          58       58              
  Lines       12546    12546              
==========================================
- Hits         8756     8748       -8     
- Misses       3790     3798       +8     

Flag	Coverage Δ
unittests	69.72% <ø> (-0.07%)	⬇️

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[qubesos-bot]

OpenQA test summary

Complete test suite and dependencies: https://openqa.qubes-os.org/tests/overview?distri=qubesos&version=4.3&build=2025030104-4.3&flavor=pull-requests

Test run included the following:

• Add Arch packaging qubes-core-admin-client#332 (QubesOS/qubes-core-admin-client@e22c531)
• Allow equal sign in feature requests #665 (ebdaf6c)
• tests: integration tests for boot mode #664 (8b3a030)
• Mark VMs with prohibit-start feature in updater qubes-desktop-linux-manager#247 (QubesOS/qubes-desktop-linux-manager@0c8493d)
• Add the feature to prohibit starting a qube qubes-manager#408 (QubesOS/qubes-manager@d7f9f5b)
• Assure Update window width does not break OpenQA qubes-desktop-linux-manager#248 (QubesOS/qubes-desktop-linux-manager@1af77ee)
• Reproduce bug in qrexec tests #661 (87f187d)
• Revert to red label while restoring on missing labels qubes-core-admin-client#335 (QubesOS/qubes-core-admin-client@6fc3cd8)
• Update firmware packages from bookworm-backports on Debian 12 qubes-core-admin-linux#184 (QubesOS/qubes-core-admin-linux@033949d)
• Add boot mode support #653 (835c90a)
• Unify method to query available terminals qubes-core-agent-linux#556 (QubesOS/qubes-core-agent-linux@5e5fc52)
• Add tests for few upcoming changes #654 (d75733d)

Installing updates failed, skipping the report!

[marmarek]

I don't see any failure with this PR (but not QubesOS/qubes-core-qrexec#186): https://openqa.qubes-os.org/tests/129865