pass active_chainstate to buildRandomProof

Summary:
This was the last use of global chainstate in the unit tests, except for the actual test fixture setup code itself (which will be removed in the two final commits of core#21866 that remain to be backported).

Depends on D11739

Test Plan:
ninja check-avalanche

Check for other use of the global chainstate in unit tests:

grep -r "g_chainman" src/test/ src/*/test
grep -r "::Chain\w*Active" src/test/ src/*/test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11742