tests: conditionalize tests based on presence of custom extensions The test harness supports injecting extensions via --extra-config-opt. However, if you do this, various tests that print state about loaded extensions fail. This commit teaches the test harness to recognize when custom extensions are loaded so that tests can use feature sniffing to conditionalize tests based on that. Differential Revision: https://phab.mercurial-scm.org/D3039