mixed-platform fixture support

[avtolstoy]

Description

Specifying different platforms in test runner config for fixture tests should now work e.g.:

{
    "fixtures": [
        {
            "name": "ble_central",
            "devices": [
                "12312312312321312323" // P2
            ]
        },
        {
            "name": "ble_peripheral",
            "devices": [
                "0a10acedXXXXXXXXXXXXXX" // Boron
            ]
        }
    ]
}