Bug 1550866 - Support try syntax '-p foo,bar,bob -u all[all]' to run Tier 2 tests on all platforms specified r=catlee

Differential Revision: https://phabricator.services.mozilla.com/D30720

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

taskcluster/taskgraph/try_option_syntax.py

@@ -602,6 +602,8 @@ class TryOptionSyntax(object):
                 return False
             tier = task.task['extra']['treeherder']['tier']
             if 'platforms' in test:
+                if 'all' in test['platforms']:
+                    return True
                 platform = attr('test_platform', '').split('/')[0]
                 # Platforms can be forced by syntax like "-u xpcshell[Windows 8]"
                 return platform in test['platforms']