HTML search: add test coverage, and introduce test module file suffix

[jayaddison]

Feature or Bugfix

• Refactoring

Purpose

• Add test coverage for search: order non-main index entries after other results #11696.
  • Index entries created using the :index: directive generally appear below similar matches in document section titles -- unless the exclamation mark (index entry priority / main flag) is set in the source reference.
• Add test coverage for partial string search matching.
• Use a .spec.js suffix for JavaScript test modules.

Detail

• N/A

Relates

• Relates to search: order non-main index entries after other results #11696.
• Scavenges test coverage from [HTML search] Add test coverage to prepare for anticipated partial-match refactoring. #12592.

[jayaddison]

Thanks!