Add proptest for breakdown-reveal aggregation

[andyleiserson]

And some general proptest cleanup and improvements. Run longer versions of the MPC proptests when EXEC_SLOW_TESTS is set.

Prompted by #1463.

[codecov]

Codecov Report

Attention: Patch coverage is 78.90295% with 50 lines in your changes missing coverage. Please review.

Project coverage is 93.78%. Comparing base (dca5be7) to head (ab147ed).
Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
...c/protocol/ipa_prf/aggregation/breakdown_reveal.rs	46.34%	44 Missing ⚠️
ipa-core/src/protocol/context/dzkp_validator.rs	66.66%	4 Missing ⚠️
ipa-core/src/test_fixture/world.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1470      +/-   ##
==========================================
- Coverage   93.87%   93.78%   -0.09%     
==========================================
  Files         235      235              
  Lines       42128    42677     +549     
==========================================
+ Hits        39546    40024     +478     
- Misses       2582     2653      +71     

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

[andyleiserson]

The bulk of the missing coverage is because I marked the OPRF version of the proptest as #[ignore]. The two versions are different enough in terms of code that it seemed worth keeping the OPRF version, but not different enough in coverage that it seemed worth the cost of running both.