linter: Add test case for custom_check.py
linter: Add test case for custom_check.py

Fixes #5406.

amanharitsh123/zulip,synicalsyntax/zulip,hackerkid/zulip,zulip/zulip,hackerkid/zulip,synicalsyntax/zulip,showell/zulip,verma-varsha/zulip,eeshangarg/zulip,eeshangarg/zulip,jackrzhang/zulip,mahim97/zulip,amanharitsh123/zulip,kou/zulip,rht/zulip,eeshangarg/zulip,tommyip/zulip,rishig/zulip,rishig/zulip,dhcrzf/zulip,rishig/zulip,timabbott/zulip,timabbott/zulip,amanharitsh123/zulip,shubhamdhama/zulip,tommyip/zulip,hackerkid/zulip,punchagan/zulip,brockwhittaker/zulip,eeshangarg/zulip,andersk/zulip,hackerkid/zulip,tommyip/zulip,shubhamdhama/zulip,tommyip/zulip,jackrzhang/zulip,jackrzhang/zulip,timabbott/zulip,rishig/zulip,shubhamdhama/zulip,mahim97/zulip,zulip/zulip,showell/zulip,eeshangarg/zulip,dhcrzf/zulip,brainwane/zulip,hackerkid/zulip,eeshangarg/zulip,shubhamdhama/zulip,zulip/zulip,punchagan/zulip,rht/zulip,verma-varsha/zulip,verma-varsha/zulip,timabbott/zulip,mahim97/zulip,Galexrt/zulip,eeshangarg/zulip,amanharitsh123/zulip,timabbott/zulip,brainwane/zulip,kou/zulip,showell/zulip,synicalsyntax/zulip,showell/zulip,kou/zulip,synicalsyntax/zulip,brockwhittaker/zulip,tommyip/zulip,andersk/zulip,timabbott/zulip,jackrzhang/zulip,brockwhittaker/zulip,dhcrzf/zulip,andersk/zulip,kou/zulip,mahim97/zulip,brainwane/zulip,zulip/zulip,shubhamdhama/zulip,punchagan/zulip,Galexrt/zulip,amanharitsh123/zulip,hackerkid/zulip,jackrzhang/zulip,kou/zulip,synicalsyntax/zulip,Galexrt/zulip,mahim97/zulip,andersk/zulip,rht/zulip,showell/zulip,dhcrzf/zulip,andersk/zulip,rishig/zulip,kou/zulip,brainwane/zulip,punchagan/zulip,punchagan/zulip,verma-varsha/zulip,synicalsyntax/zulip,verma-varsha/zulip,tommyip/zulip,zulip/zulip,shubhamdhama/zulip,timabbott/zulip,kou/zulip,brockwhittaker/zulip,showell/zulip,amanharitsh123/zulip,rht/zulip,jackrzhang/zulip,Galexrt/zulip,tommyip/zulip,brainwane/zulip,mahim97/zulip,showell/zulip,rishig/zulip,andersk/zulip,Galexrt/zulip,rht/zulip,brockwhittaker/zulip,brockwhittaker/zulip,hackerkid/zulip,verma-varsha/zulip,dhcrzf/zulip,punchagan/zulip,Galexrt/zulip,punchagan/zulip,rht/zulip,zulip/zulip,Galexrt/zulip,dhcrzf/zulip,dhcrzf/zulip,brainwane/zulip,shubhamdhama/zulip,brainwane/zulip,andersk/zulip,rishig/zulip,synicalsyntax/zulip,zulip/zulip,rht/zulip,jackrzhang/zulip