Fix add gatt by adding new **primary** services

[EasyRiderr]

The server.add_gatt() method is expected to add several services to the server, meaning the server is expected to advertise and expose these added services (see #142).

As a starting point, this PR refactors the test/test_server.py file to facilitate code reuse and adds an async def test_server_add_gatt() function to highlight this issue.

You can see the test results (before the fix is implemented) attached: trace_before.log. Tests are failing because the second characteristic (and service is not advertised, nor exposed).

On Ubuntu, BlueZ only advertise or expose the primary BLE services, so to fix the add_gatt method, I've just forced the primary parameter to True, while its default value is False. It fixes the issue #142 as you can see it test results attached:
trace_after.log

⚠️ Code has been tested only Ubuntu.

[kevincar]

@EasyRiderr Thanks for these edits!

The edits as you've written are a good workaround for now.

I think the underlying issue is the fact that the primary parameter is typed as Optional. It shouldn't be, and it should default to True on the __init__ function across all backends. This is particularly true because WinRT can only create primary services. In addition, it only makes sense to create secondary services if they can be added to the includes of the primary service, which are only available on BlueZ and CoreBluetooth. I'll make a separate issue to address this specifically, as these edits suffice for now.