Bug 1603844 - [manifestparser] Merge 'prefs' from defaults instead of overwriting them, r=gbrown

While the 'prefs' key can only exist in the DEFAULT section of manifests, this
allows parent manifests to propagate their prefs down to included ones.

Differential Revision: https://phabricator.services.mozilla.com/D57635

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

testing/mozbase/manifestparser/manifestparser/ini.py

@@ -155,6 +155,7 @@ def combine_fields(global_vars, local_vars):
     if not local_vars:
         return global_vars.copy()
     field_patterns = {
+        'prefs': '%s %s',
         'skip-if': '(%s) || (%s)',
         'support-files': '%s %s',
     }