Make sorting deterministic

[mernst]

Without this change, files __functions and __tests can appear in different order.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[tautschnig]

Thank you for the fix! I'd appreciate squashing/rebasing so that we end up with a clean git history after merging this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.77%. Comparing base (f88ea70) to head (a504540).
⚠️ Report is 4 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #9110      +/-   ##
===========================================
- Coverage    80.77%   80.77%   -0.01%     
===========================================
  Files         1712     1712              
  Lines       189814   189814              
  Branches        73       73              
===========================================
- Hits        153328   153325       -3     
- Misses       36486    36489       +3     

☔ 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.

[mernst]

I'd appreciate squashing/rebasing so that we end up with a clean git history after merging this PR.

OK, done. (I usually achieve that via squash-and-merge.)