markdown: Add unit test for render_tex.
markdown: Add unit test for render_tex.

Test for invalid path of Katex.

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