tests: Use check_dict for external_authentication_methods
tests: Use check_dict for external_authentication_methods

This is still imperfect, but the only goal for now is
to make sure that `check_list` always get a sub_validator.

zulip/zulip,brainwane/zulip,punchagan/zulip,timabbott/zulip,kou/zulip,showell/zulip,synicalsyntax/zulip,synicalsyntax/zulip,kou/zulip,showell/zulip,zulip/zulip,punchagan/zulip,hackerkid/zulip,eeshangarg/zulip,brainwane/zulip,punchagan/zulip,rht/zulip,kou/zulip,showell/zulip,hackerkid/zulip,eeshangarg/zulip,rht/zulip,hackerkid/zulip,punchagan/zulip,kou/zulip,zulip/zulip,brainwane/zulip,eeshangarg/zulip,synicalsyntax/zulip,andersk/zulip,brainwane/zulip,hackerkid/zulip,andersk/zulip,eeshangarg/zulip,andersk/zulip,brainwane/zulip,timabbott/zulip,rht/zulip,synicalsyntax/zulip,showell/zulip,timabbott/zulip,rht/zulip,hackerkid/zulip,andersk/zulip,showell/zulip,rht/zulip,andersk/zulip,timabbott/zulip,punchagan/zulip,brainwane/zulip,eeshangarg/zulip,kou/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,timabbott/zulip,hackerkid/zulip,hackerkid/zulip,synicalsyntax/zulip,showell/zulip,synicalsyntax/zulip,rht/zulip,zulip/zulip,andersk/zulip,punchagan/zulip,timabbott/zulip,timabbott/zulip,zulip/zulip,brainwane/zulip,punchagan/zulip,zulip/zulip,synicalsyntax/zulip,zulip/zulip,eeshangarg/zulip,showell/zulip,kou/zulip