Search completion; reinstate relevancy-score filter.

See the comment in the diff.

This test was in a previous version, but was dropped because with
eb0e2964fca5ef2eccc06607944df6b208b2b99f, it was (thought to be) no
longer possible to filter by relevancy score since we don't know what
the relevancy score will be.

The filter is reinstated here because we use the simple idea that,
whatever relevancy scores are assigned to completion suggestions, they
will be less than the score for a perfect suggestion.

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