fix weird error in test test_absent_delete
fix weird error in test test_absent_delete

In the function grains.setvals, when deleting a key (destructive=True),
the __grains__ var, containing the current grains was updated only if
the grains file actually contained the key. In this test, it was not the
case because the grains file is not actually used, but the current grains
var was set manually.

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