Test bug - a lib-ethers test calls getLQTYBalance instead of getLUSDBalance

[benlongstaff]

The "should close the Trove with some LUSD from another user" test in lib-ethers calls getLQTYBalance instead of getLUSDBalance.

const lusdBalance = await liquity.getLQTYBalance();

[edmulraney]

Good spot! Feel free to add a PR