Fix mock server for the tests on listing instances

For testing the element connect-dashboard-project-instances.js, a mock server is used for mocking the different calls to the platform. But none of the calls are caught and mocked by the configured sinon.fakeServer. The first call made by the element is to GET /json/cmis and an error 404 is returned. It seems the call is made before the setup of the mock server but we haven't been able to find the issue yet.

The test is available in the branch fix-NXS-6735-fix-server-calls.