test: Extract basic_stream_fields.
test: Extract basic_stream_fields.

This avoids some code duplication as well
as adding some missing fields.

We also use check_dict_only to prevent
folks from adding new fields to the
relevant events without updating these
tests.  (A bigger sweep comes later.)

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