The `-S` option resets any test variable set in an execution mode using the `-S` option

[vkarak]

How to reproduce

Create the following mode_config.py:

site_configuration = {
    'modes': [
        {
            'name': 'foo',
            'options': ['-c unittests/resources/checks/hellocheck.py', '-n ^HelloTest', '-S descr=foo_test']
        }
    ]
}

and then run the following:

reframe -C mode_config.py --mode=foo --describe | jq .[].descr

The test description is as expected:

"foo_test"

However if we set an irrelevant variable, the variable set in the mode will be lost:

reframe -C mode_config.py --mode=foo -S num_tasks=1 --describe | jq .[].descr

Then the test description is reset:

"C Hello World test"