Fix core tests on GameLoop

[Katreque]

The core tests from GameLoop are not working properly. The Sinon Spies calledOnce are returning false, even though the spied functions are called.

TempoJS/tests/classes/GameLoop.test.js

Line 85 in 337171a

test('runGameIteration should work.', async (t) => {

TempoJS/tests/classes/GameLoop.test.js

Line 105 in 337171a

test('runGameIteration should stop once stopGameIteration is called.', (t) => {

Maybe the spies are not being used properly or the runGameIteration function is not well structured.
Help is wanted since these tests assure the functioning of the engine.