[sui move] add testing function for destroying a supply

[sblackshear]

Good suggestion from @kklas

[kklas]

Use case - writing a unit test for AMM Pool https://github.com/kunalabs-io/sui-smart-contracts/blob/master/amm/move/sources/amm.move#L85. Without destroy_supply_for_testing you can only do test_scenario tests.