[4/6] test: move v2-specific tests into legacy dir

[mcous]

Stack:

• [1/6] test: refactor test_mock.py to align with v3 #311
• [2/6] test: refactor test_verify.py to align with v3 #312
• [3/6] test: refactor typing tests to align with v3 #313
• [4/6] test: move v2-specific tests into legacy dir #314
• [5/6] test: copy mock, when, verify, and typing to tests/legacy #315
• [1/3] feat: add decoy.next preview API #316

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test: move v2-specific tests into legacy dir

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (f7363f3) to head (30d83be).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #314   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           19        19           
  Lines          786       786           
  Branches        84        84           
=========================================
  Hits           786       786           

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