tests: conditionalize return code on chg in test-config.t
If there is any error while reading config, chg just returns 255 instead of 30.
It seems to me that we cannot conditionalize only return codes in output using
trailing `(chg !)` and hence used testcases.
The test was failing with chg but after this patch, it now passes.
Differential Revision: https://phab.mercurial-scm.org/D9463
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