Avoid usage of toBeTruthy/toBeFalsy in spec/frontend/ide/lib/common/model_manager_spec.js

Context

We have a frontend testing guide to avoid usage of toBeTruthy/toBeFalsy https://docs.gitlab.com/ee/development/testing_guide/frontend_testing.html#avoid-using-tobetruthy-or-tobefalsy which can be enforced via eslint-plugin-jest. However, before enabling the rule we need to update all instances of this usage in spec/frontend/ide/lib/common/model_manager_spec.js

Implementation guide

Please see strategies in parent epic