Testing: test vFuture, so we are exercising branch512

[jannotti]

We used to test CurrentVersion, now we test an old version, current version, and vFuture, which is a better test as we make development changes.

This also eliminates a funny source of flakiness, because the test randomly selects a different version to tuck into the header to test that it is detected as wrong. But if that random version has different rules about branch512, then it was causing a different failure than expected.

Test Plan

New test added for branch512 correctness

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 50.50%. Comparing base (6147ebf) to head (fc49a90).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6365      +/-   ##
==========================================
- Coverage   50.66%   50.50%   -0.16%     
==========================================
  Files         653      653              
  Lines      110377   110419      +42     
==========================================
- Hits        55918    55772     -146     
- Misses      51592    51771     +179     
- Partials     2867     2876       +9     

☔ 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.