test_events: Add test for `do_change_logo_source` event.
test_events: Add test for `do_change_logo_source` event.

This adds tests for `do_change_logo_source` event in both day as well as
night mode in `test_event.py`.

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