[HTML search] tests: bugfix: correction for test index fixture format

[jayaddison]

Feature or Bugfix

• Bugfix

Purpose

• Make the tests/js/searchtools.js test file more representative of the search index format used by the Sphinx client-side JS code.

Detail

• Ranking of search results relies on scoring, and Sphinx exact-match queries on terms (arbitrary words) are intended to score 5, while partial-matches score 2 per term (ref).
• Multi-word searches such as datetime timedelta are supported by Sphinx; internally the JavaScript on the page tokenizes these into multi-term queries (['datetim', 'timedelta']).
• The search index itself only contains single-word terms.
• Each term in the index is mapped to a list of documents where the term is found, and these document-ids are zero-indexed (0, 1, 2, ...).
• 🐛 The tests/js/searchtools.js file has an incorrect representation of the terms entry in the index, instead mapping a search term to a single document-id.

Relates

• Resolves Test suite: search: test fixture does not accurately represent the JS search index format #12028.

[jayaddison]

My mistake; single-document terms are indexed as flat integer values, as noted by @wlach here. Apologies for the review-ping noise.