Add integration tests for xmvn-subst

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 70.91%. Comparing base (b59ee70) to head (f4cd52f). Report is 3 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #347 +/- ## ========================================= Coverage 70.91% 70.91% Complexity 314 314 ========================================= Files 106 106 Lines 3424 3424 Branches 504 504 ========================================= Hits 2428 2428 Misses 790 790 Partials 206 206 ```

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