Improve QPU selection in QPU tests

[shahidee44]

Prerequisites

Before raising this issue, I have already checked that I am:

• running the latest version
• made sure that this issue has not already been filed

Describe the bug

The name of the QPU used for the unittest is hardcoded. To change to automated device name selection.
(https://github.com/entropicalabs/openqaoa/blob/main/tests/test_qpu_qiskit.py#L504)

Expected behavior

In the scenario where ibmq_perth gets decommissioned, the code will not work. Also this code might not work if the device is not accepting jobs. (Although technically the Device Object should already reject this device if it isn't Online, even before the Job is sent.)