Add tests for PayOverXcm (#7319)

* PayOverXcm: Add test skeleton

* PayOverXcm: Add tests checking PayOverXcm generates correct message

* Add PayOverXcm and salary pallet integration test

* Fix XCM execution issue

* Fix documentation issues

* Add missing period at the end of comment

* Fix mock problems

* Add missing licenses

* Fix lints

* Fix more lints

* Add docs to mock

* Add transfer assertions

* Remove magic numbers

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Co-authored-by: parity-processbot <>