bug 1596963 - run delegated credentials xpcshell tests serially r=kjacobs

The delegated credentials xpcshell tests use the TLS test server framework,
which currently uses a hard-coded port, so these tests need to run serially.

Differential Revision: https://phabricator.services.mozilla.com/D53301

--HG--
extra : moz-landing-system : lando

security/manager/ssl/tests/unit/xpcshell.ini

@@ -109,7 +109,9 @@ run-sequentially = hardcoded ports
 # this test doesn't apply.
 skip-if = toolkit == 'android'
 [test_delegated_credentials.js]
+run-sequentially = hardcoded ports
 [test_delegated_credentials_weak.js]
+run-sequentially = hardcoded ports
 [test_der.js]
 [test_enterprise_roots.js]
 # This feature is implemented for Windows and OS X. However, we don't currently