tests: fix flaky expirer unit test

[mmat11]

Fixes:

• https://github.com/open-telemetry/opentelemetry-ebpf-instrumentation/actions/runs/22650194232/job/65647935550
• https://github.com/open-telemetry/opentelemetry-ebpf-instrumentation/actions/runs/22633504801/job/65591064623
• https://github.com/open-telemetry/opentelemetry-ebpf-instrumentation/actions/runs/22623838632/job/65556212921

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: mmat11 / name: Mattia Meleleo (9949d29)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.90%. Comparing base (b2244db) to head (9949d29).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1451      +/-   ##
==========================================
- Coverage   43.93%   43.90%   -0.03%     
==========================================
  Files         313      313              
  Lines       34213    34213              
==========================================
- Hits        15030    15020      -10     
- Misses      18207    18214       +7     
- Partials      976      979       +3     

Flag	Coverage Δ
integration-test	21.55% <ø> (-0.16%)	⬇️
integration-test-arm	0.00% <ø> (ø)
integration-test-vm-x86_64-5.15.152	?
integration-test-vm-x86_64-6.10.6	?
k8s-integration-test	2.29% <ø> (+<0.01%)	⬆️
oats-test	0.00% <ø> (ø)
unittests	44.89% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[grcevski]

LGTM! Thanks so much!