Fix tests/jobscript/05-global-config.t for use with platforms

[wxtim]

Earlier work on platforms has broken this test:

We are not intending to have this globalconfig work in Cylc 8. The best this test should give is a helpful warning, but that warning should already have been given elsewhere. More likely we need to re-write this in a platformsy way.

[oliver-sanders]

@datamel you get to close this one the easy way!

[wxtim]

This issue can be closed at the test in question has been replaced by tests/unit/test_job_file.py