Bug 1888394: Update tests within accessible/ to work with https-first enabled, r=Jamie

Differential Revision: https://phabricator.services.mozilla.com/D206006

accessible/tests/browser/e10s/browser.toml

@@ -31,6 +31,7 @@ prefs = [
 ["browser_caching_description.js"]
 
 ["browser_caching_document_props.js"]
+prefs = ["dom.security.https_first=false"]
 
 ["browser_caching_domnodeid.js"]