gistfile1.txt

// Works fine
const { SearchService } = ChromeUtils.import(
  "resource://gre/modules/SearchService.jsm"
);

but

const { SearchTestUtils } = ChromeUtils.import(
  "resource://testing-common/SearchTestUtils.jsm"
);

gives 

// [Exception... "Component is not available"  nsresult: "0x80040111 (NS_ERROR_NOT_AVAILABLE)"  
   location: "JS frame :: file:///Users/dale/src/search-helper-extension/extension/experiments/searchengines/api.js :: 
   <TOP_LEVEL> :: line 7"  data: no]