mattermost: Fix test flake in test_convert_channel_data.
mattermost: Fix test flake in test_convert_channel_data.

So the reason 38f8cf612c9245c3f81ec94ae780ab78165ae186 seems
to be flaking is because the value of harry id switches between
1 and 2 in Xenial while in Bionic it would be fixed at 2. The
reason behind this is that Bionic ships with Python3.6 which
preserves dict insert order while Python3.5 that ships with Xenial
dont preserve the order. In initialize_stream_membership_dicts
we iterate user_data_map dict and the order in which the iteration
happens affects the ID of the users.

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