Add test asserting that rules should be unique

[LDiazN]

closes #182

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.31%. Comparing base (c8fcc72) to head (ba819e5).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #183      +/-   ##
==========================================
+ Coverage   83.24%   83.31%   +0.07%     
==========================================
  Files          89       89              
  Lines        5788     5813      +25     
==========================================
+ Hits         4818     4843      +25     
  Misses        970      970              

Flag	Coverage Δ
oonidata	77.87% <ø> (ø)
oonipipeline	86.03% <ø> (+0.09%)	⬆️

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

☔ View full report in Codecov by Harness.
📢 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.