test_zulint_custom_rules: Fix open mock.
test_zulint_custom_rules: Fix open mock.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

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