Bug 1561994 - Fix browser_aboutCertError_mitm.js so it passes when security.enterprise_roots.enabled is enabled by default. r=keeler

Turn off security.enterprise_roots.enabled for this test since that's
what it expects.

Differential Revision: https://phabricator.services.mozilla.com/D36334

--HG--
extra : rebase_source : 5904a8668cfaa7654895239e7698cac7338ffb51

browser/base/content/test/about/browser_aboutCertError_mitm.js

@@ -61,6 +61,7 @@ add_task(async function checkMitmAutoEnableEnterpriseRoots() {
     [PREF_MITM_PRIMING, true],
     [PREF_MITM_PRIMING_ENDPOINT, UNKNOWN_ISSUER],
     [PREF_MITM_AUTO_ENABLE_ENTERPRISE_ROOTS, true],
+    [PREF_ENTERPRISE_ROOTS, false],
   ]});
 
   let browser;