test-override: Assert that only the expected term is negated

We weren't distinguishing here between overrides that should have been
negated (xdg-documents) and overrides that should not have been negated
(everything else).

Signed-off-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit 4e3d1d8b7bbd4c0611b6bb44f67c6ad1734d6b7d)