From bug 1584920, it looks like when we add a new locale, the modern configuration is causing the new locale to have Amazon, Bing and eBay added to it.

The tests are catching this and causing failures, e.g.

TEST-UNEXPECTED-FAIL | xpcshell-1.ini:toolkit/components/search/tests/xpcshell/searchconfigs/test_amazon.js | test_searchConfig_amazon - [test_searchConfig_amazon : 149] Should not be available for region: "ad" locale: "bo" - false == true
TEST-UNEXPECTED-FAIL | xpcshell-1.ini:toolkit/components/search/tests/xpcshell/searchconfigs/test_bing.js | test_searchConfig_bing - [test_searchConfig_bing : 149] Should not be available for region: "ad" locale: "bo" - false == true
TEST-UNEXPECTED-FAIL | xpcshell-1.ini:toolkit/components/search/tests/xpcshell/searchconfigs/test_ebay.js | test_searchConfig_ebay - [test_searchConfig_ebay : 149] Should not be available for region: "ad" locale: "bo" - false == true

Could be something to do with bug 1575554, though that should only affect things if the region isn't specified.

I've just taken a deeper look - this isn't the modern configuration, but the legacy one. The legacy one is adding these engines to the new locales by default, and that's something we don't actually want to do here, aiui.