test1.cs

var level = Substitute.For<ILevel>();
var buildings = Substitute.For<IBuildings>();

// test subject: 
var build = new BuildController(null,buildings,level);

// smoke test
Assert.AreEqual(0, build.GetCurrentBuildCount());

// assert that `GetCurrent` was exactly called once
level.ReceivedWithAnyArgs(1).GetCurrent();