email_mirror: Add a test for sending to a private stream.
email_mirror: Add a test for sending to a private stream.

This verifies an important case.  We still have an open bug for why in
some production environments, the email_gateway_bot seems to not be
tagged as an API super user (resulting in this code path not working).

timabbott/zulip,rishig/zulip,brainwane/zulip,eeshangarg/zulip,rht/zulip,hackerkid/zulip,punchagan/zulip,kou/zulip,brainwane/zulip,punchagan/zulip,andersk/zulip,showell/zulip,eeshangarg/zulip,dhcrzf/zulip,brainwane/zulip,brainwane/zulip,timabbott/zulip,zulip/zulip,jackrzhang/zulip,shubhamdhama/zulip,zulip/zulip,showell/zulip,kou/zulip,tommyip/zulip,jackrzhang/zulip,rishig/zulip,zulip/zulip,rht/zulip,hackerkid/zulip,kou/zulip,rishig/zulip,jackrzhang/zulip,eeshangarg/zulip,synicalsyntax/zulip,synicalsyntax/zulip,timabbott/zulip,rht/zulip,rht/zulip,hackerkid/zulip,punchagan/zulip,shubhamdhama/zulip,dhcrzf/zulip,rht/zulip,zulip/zulip,shubhamdhama/zulip,showell/zulip,punchagan/zulip,dhcrzf/zulip,jackrzhang/zulip,rishig/zulip,brainwane/zulip,rht/zulip,timabbott/zulip,punchagan/zulip,eeshangarg/zulip,shubhamdhama/zulip,andersk/zulip,synicalsyntax/zulip,andersk/zulip,synicalsyntax/zulip,tommyip/zulip,shubhamdhama/zulip,timabbott/zulip,andersk/zulip,rht/zulip,andersk/zulip,eeshangarg/zulip,shubhamdhama/zulip,shubhamdhama/zulip,punchagan/zulip,dhcrzf/zulip,rishig/zulip,hackerkid/zulip,hackerkid/zulip,synicalsyntax/zulip,kou/zulip,kou/zulip,tommyip/zulip,rishig/zulip,showell/zulip,synicalsyntax/zulip,andersk/zulip,dhcrzf/zulip,tommyip/zulip,eeshangarg/zulip,punchagan/zulip,hackerkid/zulip,brainwane/zulip,rishig/zulip,showell/zulip,jackrzhang/zulip,andersk/zulip,jackrzhang/zulip,zulip/zulip,synicalsyntax/zulip,dhcrzf/zulip,timabbott/zulip,showell/zulip,zulip/zulip,tommyip/zulip,brainwane/zulip,timabbott/zulip,kou/zulip,showell/zulip,hackerkid/zulip,eeshangarg/zulip,tommyip/zulip,zulip/zulip,jackrzhang/zulip,dhcrzf/zulip,kou/zulip,tommyip/zulip