Use context managers as suggested by PyLint 2.8.2

Use context managers as suggested by PyLint 2.8.2

PyLint 2.8.2 reports the following suggestions for two Python scripts
used in the system test suite:

    ************* Module tests_rndc_deadlock
    bin/tests/system/addzone/tests_rndc_deadlock.py:71:4: R1732: Consider using 'with' for resource-allocating operations (consider-using-with)
    ************* Module tests-shutdown
    bin/tests/system/shutdown/tests-shutdown.py:68:4: R1732: Consider using 'with' for resource-allocating operations (consider-using-with)
    bin/tests/system/shutdown/tests-shutdown.py:154:8: R1732: Consider using 'with' for resource-allocating operations (consider-using-with)

Implement the above suggestions by using
concurrent.futures.ThreadPoolExecutor() and subprocess.Popen() as
context managers.