tests: Mock print() for management command tests.
tests: Mock print() for management command tests.

This avoids spam in test-backend output.

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